The Milky Way is a spiral galaxy that encompasses our solar system and planet Earth. It is characterized by a prominent band of bright stars extending from its center and traversing the galaxy. Its name, Milky Way, is derived from the Latin term via lacteal, meaning “milky way,” which is itself a translation of the ancient Greek term ϰύϰλος γαλαξίας, meaning “milky circle.” Although the mass of the Milky Way has been previously determined, advancements in scientific understanding have allowed for further refinement of this value.
The Milky Way galaxy is approximately 100,000 light-years in diameter and has an estimated thickness of around 1,000 light-years. It is home to a staggering number of stars, estimated to be between 200 and 400 billion. As of January 2009, the mass of the Milky Way was calculated to be 6-10,42 kilograms, a mind-boggling figure. Interestingly, the majority of the galaxy’s mass is not found within its stars and planets, but rather in an invisible halo composed of dark matter.
As per the latest issue of The Astrophysics Journal, new data has been released regarding the mass of the Milky Way, which now stands at about 9.5-10 41 kilograms. This is believed to be the most accurate measurement to date, although it is difficult for the average person to verify these findings. The calculations involved a highly intricate mathematical process, along with complex statistical analysis, to reach the final result. Astronomers utilized hierarchical Bayesian analysis and direct measurements of globular star cluster velocities in order to obtain this information.
For a long time, scientists have had knowledge on how to determine the weight of the Sun through observing its gravitational impact on Earth. A similar procedure is carried out to determine the weight of the entire galaxy, by measuring the gravitational force of globular clusters containing stars. The most recent calculation of the mass of the Milky Way takes into account various components such as stars, planets, moons, dust, and dark matter.
| Galaxy | |
 The Milky Way is a spiral galaxy with a junction. It is believed that there is a supermassive black hole called Sagittarius A* at its center. Two of its four arms are dominant. | |
| SBbc (spiral junction galaxy) [1] | |
| 100,000 light-years [2] | |
| 3000 light-years (bulge) [3] 1000 light-years (disk) [2] | |
| 200-400 billion [2] [4] | |
| 4.8 × 10^11 M ⊙ > [5] | |
| 13.2 billion years [6] | |
| 26,000 ± 1,400 light-years | |
| 225-250 million years | |
| 220-360 million years [7] | |
| 100-120 Ma [8] [7] | |
| 552 km/s [9] | |
| 550 km/s | |
| 2.1 × 10^10 L ⊙ or 8.3 × 10^36 W [10] | |
| -21.00 ± 0.38 [10] | |
The Milky Way, along with the Andromeda Galaxy (M31), the Triangle Galaxy (M33), and over 40 satellite dwarf galaxies – both belonging to the Milky Way and Andromeda – comprise the Local Group of galaxies[15], which is a constituent of the Local Supergroup (Virgo Supergroup)[16] .
Origin of the Name [ edit edit code ]
The name Milky Way is widely used in Western culture and comes from the Latin phrase via lactea, which means “milky way”. This Latin phrase is a translation of the original Greek term ϰύϰλος γαλαξίας, which means “milk circle” [17]. The title Galactica is also derived from the Greek word γαλαϰτιϰτιϰός, which means “milky”. According to ancient Greek mythology, Zeus decided to grant immortality to his son Heracles, who was born to a mortal woman. To achieve this, Zeus placed Heracles next to his sleeping wife Hera, so that Heracles could drink her divine milk. When Hera woke up and realized what was happening, she pushed Heracles away, causing the splashing milk to transform into the Milky Way.
In the Soviet astronomy school, our Galaxy was commonly referred to as the Milky Way galaxy [18] or the Milky Way system. The term “Milky Way” [19] was used to describe the visible stars that make up the Milky Way as seen by an observer.
There are numerous other names for the Milky Way in various languages. The term “path” is often retained, but different epithets are used in place of “Milky Way”. For instance, in Arabic, it is referred to as a flour path, which is said to have been created from flour that was scattered from a sack with holes while it was on a cart.
Structure of the Galaxy
Size of the Galaxy
The Milky Way has a diameter of approximately 30 thousand parsecs (equivalent to about 100,000 light-years or 1 quintillion kilometers), with an estimated average thickness of around 1,000 light-years. Researchers at the Canary Institute of Astrophysics have determined, after conducting statistical analyses of data from the APOGEE and LAMOST missions, that the diameter of the Milky Way’s disk is about 200,000 light-years [20].
Quantity of stars [ edit edit code ] .
The galaxy is home to approximately 200 to 400 billion stars, as per current calculations. These stars are primarily distributed in a flat disk shape[21] .
The Milky Way Galaxy also consists of 25 to 100 billion brown dwarfs [22] .
Total mass [ edit edit code ]
The majority of the Galaxy’s mass is not comprised of stars and interstellar gas, but rather the non-luminous halo of dark matter, making it quite challenging to precisely determine the Milky Way’s exact mass.
As of January 2009, the mass of the Galaxy was estimated to be 3⋅10 12 solar masses [23] , or 6⋅10 42 kg.
In May 2016, astrophysicists from Canada published an estimate stating that the Galaxy’s mass is only 7⋅10 11 solar masses [24] .
In 2019, scientists utilized data from the Gaia and Hubble missions to determine that the mass of the Milky Way, within a radius of 129,000 light-years from the Galactic center, is approximately 1.5 ⋅10 12 solar masses [25].
Disk [ edit edit code ].
It wasn’t until the 1980s that astronomers proposed the idea that the Milky Way is a barred spiral galaxy [26], rather than a regular spiral galaxy. This hypothesis was confirmed in 2005 by the Lyman Spitzer Space Telescope, which revealed that the central bulge of our galaxy is larger than previously believed [27].
According to scientists, the galactic disk, which protrudes in various directions near the galactic center, is estimated to have a diameter of approximately 100,000 light-years [28]. In comparison to the halo, the disk exhibits a noticeably faster rotation. The rotation rate of the disk is not constant at different distances from the center. It rapidly increases from zero at the center to 200-240 km/s at a distance of 2,000 light-years, then slightly decreases, increases again to approximately the same value, and then remains relatively constant. By studying the characteristics of the disk’s rotation, researchers were able to estimate its mass, which turned out to be 150 billion times greater than the mass of the disk (M ⊙).
The S-shaped stellar disk in the outer regions of the Milky Way is progressively twisted into a spiral structure [29] [30] [31].
Researchers at the Australian National University have found that the magnetic field strength in the Milky Way disk is estimated to be between 15 and 20 μG (microgauss) [32].
The central regions of the Milky Way are distinguished by a dense concentration of stars: each cubic parsec near the core contains numerous stars. The distances between these stars are significantly smaller compared to those in the vicinity of the Sun. Similar to other galaxies, the mass distribution in the Milky Way is such that the majority of stars in the Galaxy have orbital velocities that are not significantly influenced by their distance from the core. As one moves further from the core towards the outer regions, the typical orbital velocity of stars ranges from 210-240 km/s. This velocity distribution, which is not observed in the solar system where different orbits have distinct orbital velocities, serves as one indication of the presence of dark matter.
The galactic jumper, estimated to be approximately 27,000 light-years in length[26], traverses the galaxy’s center at an inclination of 44 ± 10 degrees relative to the line connecting our Sun and the galactic center. Comprising predominantly of aging red stars, the jumper is encompassed by a ring named the Five Kiloparsec Ring. This ring houses the majority of the galaxy’s molecular hydrogen and serves as an active hub for star formation within our own galaxy. From the perspective of the Andromeda Galaxy, the Milky Way’s galactic junction would appear as a luminous segment [39].
In 2016, scientists from Japan’s astrophysics community made a groundbreaking announcement regarding the identification of a second enormous black hole situated at the heart of our galaxy, the Milky Way. This particular black hole is positioned approximately 200 light-years away from the galactic center. Remarkably, the observed celestial entity, encompassed by a cloud, inhabits a spatial region measuring 0.3 light-years in diameter, boasting a mass equivalent to 100,000 times that of our sun. Despite extensive research and analysis, the precise characteristics and classification of this enigmatic object remain uncertain, with scientists pondering whether it is indeed a black hole or an entirely distinct astronomical phenomenon [40].
In 2018, the Chandra X-ray Space Laboratory observed the Galactic Center and identified 12 double systems that emit low-mass X-rays. One of the components in these systems is likely a stellar-mass black hole. It is estimated that there could be 10-20 thousand black holes located 1 parsec (3.26 light-years) away from the supermassive black hole associated with the compact radio source Sagittarius A* [41].
Halo [edit edit code].
Illumination [edit modify code].
While the galactic disk contains gas and dust, which creates difficulties for visible light to pass through, there are no such components in the spheroidal component. The disk is where active star formation takes place, particularly in the spiral arms, which are regions of higher density. In the halo, star formation has ceased. The disk is also where scattered clusters are predominantly found. The majority of our galaxy’s mass is believed to be composed of dark matter, forming a dark matter halo with a mass of approximately 600-3000 billion M ⊙. This dark matter halo is concentrated towards the galaxy’s center [45].
The stars and star clusters in the halo move in highly elongated orbits around the galactic center. As the rotation of individual stars is somewhat random (meaning that the velocities of neighboring stars can have any direction), the halo as a whole rotates at a very slow pace.
Luminosity [ edit edit code ].
The estimated luminosity of the Milky Way is 2.1 +0.9
-0.6 ⋅10 10 solar luminosities, which is equivalent to 8.3 +3.5
-2.4 ⋅10 36 W. The absolute stellar magnitude of the Milky Way is -21.00 +0.38
−0.37 [10].
Discovery History [ edit edit code ]
Most celestial objects are organized into various rotating systems. For instance, the Earth’s moon orbits around the Earth, while the satellites of the giant planets form their own systems with multiple bodies. At a higher level, the Earth and the other planets revolve around the Sun. This leads to the question: could the Sun also be part of an even larger system?
The English astronomer William Herschel conducted the first documented study on this matter in the 18th century. He conducted a star count in various regions of the sky and made a remarkable discovery – a prominent circle in the sky, later known as the galactic equator, which bisects the sky into two equal parts and exhibits the highest concentration of stars. Additionally, Herschel observed that the proximity of a region to this circle directly correlates with the number of stars present. Furthermore, it was determined that the Milky Way resides on this celestial circle. Based on these findings, Herschel hypothesized that all the stars we observe are part of a massive star system that is flattened around the galactic equator.
Position of the Sun within the Milky Way Galaxy [ edit edit code ]
Based on recent scientific calculations, the Sun is located approximately 27,000 ± 1,400 light-years away from the galactic center. Initial estimates had suggested that our star would be positioned around 35,000 light-years from the galactic junction. This indicates that the Sun is situated closer to the outer edge of the galaxy’s disk rather than its center. Together with other celestial bodies, the Sun orbits around the galactic center at a velocity of 220-240 km/s [46] , completing one full revolution approximately every 200 million years. Consequently, throughout its entire existence, the Earth has completed no more than 30 orbits around the central region of the Milky Way Galaxy.
Within the proximity of our Sun, it is feasible to observe two spiral arms that are approximately 3 thousand light years away from us. These regions have been designated as the Sagittarius arm and the Perseus arm based on the constellations in which they are detected. The Sun is positioned near the center point between these two spiral branches. However, there is another arm known as the Orion arm that passes through the constellation of Orion, which is relatively close to us in comparison to galactic standards. This arm is not as distinctly defined, yet it is regarded as a subsidiary branch of one of the primary spiral arms of our Galaxy.
The Sun’s speed of rotation around the center of the Galaxy closely matches the speed of the formation of the spiral arm. This is not typical for the entire Galaxy, as the spiral arms rotate at a constant angular velocity, similar to spokes on a wheel, while the stars move in a different pattern [47]. As a result, most of the stars in the disk move in and out of the spiral arms. The only location where the velocities of the stars and spiral arms align is known as the corotational circle, and it is within this circle that the Sun is positioned.
For our planet, this factor is of utmost significance since the spiral arms of the galaxy are home to intense phenomena that generate a potent radiation which poses a threat to all forms of life. No atmosphere, no matter how robust, can shield against this destructive force. Nevertheless, Earth is fortunate to reside in a relatively tranquil region within the Milky Way, having evaded these cosmic disasters for hundreds of millions, if not billions, of years. It is possible that this fortuitous positioning has contributed to the emergence and endurance of life on our planet.
There could potentially be undiscovered dwarf galaxies that are dynamically associated with the Milky Way, as demonstrated by the identification of nine new Milky Way satellites in a relatively small section of the night sky in 2015 [48]. Additionally, there are dwarf galaxies that have already been incorporated into the Milky Way, like Omega Centauri [49].
In 2014, scientists revealed that the majority of the Milky Way’s satellite galaxies are actually arranged in a large disk and orbiting in the same direction [50]. This discovery was unexpected, as per standard cosmology, satellite galaxies should form within the halo dimensions of dark matter and should be widely dispersed, moving in random directions. The reason for this inconsistency has yet to be fully explained [51].
Evolution and the future [ edit edit code ].
A team of astronomers has recently conducted a study on the motion of 30,000 galaxies, including the stars in our very own Milky Way galaxy. The findings of this study suggest that approximately 10 billion years ago, the Milky Way experienced a significant event – a merger with a larger galaxy known as Gaia-Enceladus. This merger resulted in the formation of a thick disk and caused the Milky Way to adopt an inflated shape [52]. Gaia-Enceladus, which was about 10 times smaller than the Milky Way at that time, played a crucial role in this cataclysmic event, as the size ratio between the two galaxies was 1 to 4. It is worth noting that the Milky Way itself was much smaller back then. In terms of mass, Gaia-Enceladus was slightly more massive than the present-day Small Magellanic Cloud [53] [54].
Scientists from the University of California, Riverside (USA) have calculated that approximately 1 billion years ago, the dwarf galaxy in Kiel, known as the Dwarf Galaxy in the constellation Furnace, along with a few other ultra-faint dwarf galaxies, were actually satellites of the Large Magellanic Cloud, not the Milky Way [55] [56].
While it is possible for our Galaxy to collide with other galaxies, including the Andromeda Galaxy, which is a significantly larger galaxy [57], making specific predictions is currently impossible due to the unknown transverse velocity of extragalactic objects.
Based on data released in September 2014, one model predicts that in 4 billion years, the Milky Way will “merge with” the Large and Small Magellanic Clouds, and in 5 billion years, it will be assimilated by the Andromeda Nebula [58]. Alternatively, according to different calculations, the galaxies will collide at an angle in 4.5 billion years [59].
According to calculations made by scientists at the Durham University Institute of Computational Cosmology, the Large Magellanic Cloud, which is currently moving away from the Milky Way, will reverse its trajectory in approximately 1 billion years and begin heading towards the center of our galaxy. This will lead to a merger between the two entities lasting around 1.5 billion years. Simultaneously, the central object of our galaxy, Sagittarius A*, will experience a tenfold increase in size. The collision between the Large Magellanic Cloud and the Milky Way is expected to occur in approximately 2 billion years, potentially resulting in the expulsion of our solar system into intergalactic space [60] [61] [62].
Google has created a unique project called 100,000 Stars to provide a visual representation of the Milky Way galaxy.
Move away from the lights of the city, gaze up at the heavens, and witness one of the marvels of the universe firsthand – the radiant strip of stars that constitutes our very own Milky Way galaxy. Our celestial dwelling is expansive and magnificent, yet its true weight remains a mystery. Despite years of computations, astronomers still possess only rough approximations – ranging from 700 billion to 2 trillion solar masses.
There are various ways to measure the mass of other galaxies, such as employing the phenomenon of gravitational lensing. The basic idea behind this approach is that gravity affects all objects, including light particles known as photons. While the gravitational impact is usually negligible for small entities like planets, it becomes more pronounced for stars and galaxies, as they can significantly bend light. The degree of bending corresponds to the mass of the object, enabling researchers to determine its weight.
Another, less precise, method involves studying the rotation of stars around the center of a galaxy. By observing the centripetal acceleration of a star located at the galaxy’s periphery, scientists can calculate the mass of the matter within its orbit. Although this approach is simpler, it is not as accurate as gravitational lensing.
A gravitational lens is a clear example of how light bends under the influence of gravity.
However, it is important to note that the aforementioned methods can only be applied when observing a galaxy from outside. When observing from within, the bending of light becomes imperceptible, and it becomes challenging to locate and measure the speed of a star at the galaxy’s edge (especially considering that the solar system itself orbits the center of the Milky Way). Astrophysicist Ekta Patel of Arizona State University in Tucson compares this situation to attempting a national census without access to the internet and without leaving your own city.
It is not any easier, either, because the majority of the galaxy (in terms of mass) is not visible. According to Patel, dark matter – a mysterious substance that does not emit any light – constitutes about 85% of our Milky Way. So, simply calculating the mass of all the stars in our galaxy would yield a result that is very far from the truth.
Therefore, Patel explains that astrophysicists typically rely on the gravity equations formulated by Isaac Newton over 300 years ago. These equations inform us that the speed and distance at which a smaller celestial body orbits a larger one is connected to the mass of the larger body.
According to a study published in the Astrophysical Journal, one approach is to observe dwarf galaxies located hundreds of thousands of light-years away from our planet. These galaxies follow a similar rotational pattern as planets orbiting around stars. However, there are certain challenges associated with these satellite galaxies. As Patel explained, “These galaxies have been in their orbits for billions of years,” which implies that their movement is barely noticeable over a span of a few years, making it difficult for researchers to accurately determine their orbital velocity.
Dwarf satellite galaxies surrounding the Milky Way.
Patel and her colleagues conducted a novel approach to measure the mass of galaxies in their research. They utilized simulations on a supercomputer to create virtual galaxies with small satellite galaxies, taking into consideration various aspects of the actual universe. Subsequently, they compared approximately 90,000 of these simulated satellite galaxies with data from nine genuine galaxies that orbit the Milky Way. By selecting the simulated satellite galaxies that closely matched the orbital characteristics of the Milky Way’s real satellite galaxies, the team was able to estimate the mass of the main galaxy.
Patel states that having knowledge of the mass of our galaxy would be beneficial for numerous astronomers. Initially, it would enable them to more accurately calculate the paths of satellite galaxies as they are reliant on the mass of the Milky Way. The greater the weight of the galaxy, the greater the number of satellites it possesses, and currently approximately 50 galaxies have been discovered revolving around the Milky Way. Since the precise weight of our galaxy is unknown, scientists cannot be certain of the exact number of satellite galaxies that remain undiscovered. Ultimately, determining the true weight of the Milky Way would assist researchers in understanding the ratio of dark matter to ordinary matter within its mass.
Patel anticipates that future investigations and improved data will eventually provide the necessary precision. “I believe it is a matter of the next 10 or 20 years,” she expressed.
The Galactica (from the Greek word Γαλαξίας – Milky Way) is a cluster of stars, interstellar gas, dust, and dark matter that are held together by gravity. Every object within galaxies moves in relation to a central point of mass.
Galaxies are objects that are located very far away, with the closest one typically being measured in megaparsecs and the farthest one being measured in units of redshift z. Due to their immense distance, only three galaxies can be observed with the naked eye: Andromeda (visible in the northern hemisphere), the Large Magellanic Cloud, and the Small Magellanic Cloud (both visible in the southern hemisphere). It wasn’t until the early 20th century that scientists were able to resolve the images of individual stars within galaxies. By the early 1990s, there were only about 30 galaxies in which individual stars could be seen, and all of them belonged to the local group. However, with the launch of the Hubble Space Telescope and the development of 10-meter ground-based telescopes, the number of galaxies in which individual stars can be observed has significantly increased.
Galaxies exhibit a wide range of characteristics, including spherical elliptical galaxies, disk spiral galaxies, bar galaxies, dwarf galaxies, irregular galaxies, and more. In terms of numerical values, their mass can range from 10^7 to 10^12 solar masses, and their diameter can vary from 5 to 50 kiloparsecs [1].
One unresolved mystery surrounding galaxy structure is the presence of dark matter, which is only detected through its gravitational interaction. It can account for up to 90% of a galaxy’s total mass or be completely absent, as seen in dwarf galaxies [2].
Galaxies are not evenly spread out throughout the vast expanse of space. In certain regions, clusters of galaxies can be found close together, while in other areas, there may be a complete absence of galaxies, even the tiniest ones known as woads. The exact count of galaxies in the observable portion of the Universe remains uncertain, but it is estimated to be around 10^11 [3].
The term “galactica” (Greek γαλαξίας – milky) originated from the Greek name for our Galaxy (kyklos galaktikos meaning “milky ring” – as a way to describe an observed phenomenon in the night sky) [4]. When astronomers speculated that various celestial objects believed to be spiral nebulae might actually be massive clusters of stars, these objects became known as “island universes” or “star islands”. However, once it was discovered that these objects were similar to our own Galaxy, both terms fell out of use and were replaced by the term “galaxy”.
Observations [ edit edit code ].
Galaxies possess a multitude of crucial integral traits [1] (excluding outliers):
| Diameter D25 | Photometry | 5-50 kpc | 30 kpc |
| Radial disk scale R0 | Photometry | 1-7 kpc | 3 kpc |
| Stellar disk thickness | Photometry of disks observed “from the edge” | 0.3-1 kpc | 0.7 kpc |
| Luminosity | Photometry | 10 7 -10 11 Lʘ | 5×10 10 Lʘ |
| Mass M25 within D25 | Measurement of gas and/or stellar velocities by the Doppler effect | 10 7 -10 12 Mʘ | 2×10 11 Mʘ |
| Relative mass of gas Mgas/M25 within D25 | Measurement of neutral and molecular hydrogen line intensities | 0.1—30 % | 2 % |
| Rotational velocity V of the outer regions of galaxies | Measurement of gas and/or stellar velocities by Doppler effect | 50-300 km/s | 220 km/s (for the vicinity of the Sun) |
| Rotation period of the outer regions of galaxies | Measurements of gas and/or stellar velocities by the Doppler effect | 10 8 -10 9 years | 2×10 8 (for the vicinity of the Sun) |
| Mass of the central black hole | Measurements of stellar and gas velocities near the core; empirical dependence on the central dispersion of stars | 3×10 5 -3×10 9 Mʘ | 4×10 6 Mʘ |
Main article: Distance scale in astronomy
The measurement of the distance from the observer to a galaxy is not involved in any processes occurring within the galaxy. The necessity for knowledge about the distance to a galaxy arises in various situations, such as the identification of understudied phenomena like gamma-ray bursts, the study of the entire universe, the examination of galaxy evolution, the determination of galaxy mass and size, and so on.
All the methods used to determine the distance to a galaxy, which are more or less dependent on models, can be categorized into two types: measurement through an object located within the galaxy, where the distance to the object is negligibly different from the distance to the galaxy itself, and measurement through redshift.
The initial technique used in determining distances is known as the photometric method, which involves utilizing standard candles with known luminosity. By applying the following formula, the distance can be calculated:
where m represents the apparent stellar magnitude, М denotes the absolute stellar magnitude, and R indicates the distance measured in parsecs. Currently, the following standard candles are employed for this purpose [5]:
- Cepheids, which allow us to determine their luminosity based on their pulsation period. These were the first objects used to measure distances to other galaxies.
- Type Ia supernovae, which played a crucial role in discovering the accelerated expansion of the Universe in the 1990s.
- Red giants
- Supergiants
The second technique relies on Hubble’s empirical law and is more model-dependent compared to the previous method.
The Hubble constant, denoted as H0, plays a crucial role in various astrophysical models. In the widely accepted ΛCDM model, which shares the same Hubble constant, any notable deviation would likely occur at z~10, thus making it highly dependent on the specific model.
Furthermore, there exist several methods that heavily rely on the chosen model [5]:
- Utilizing the Sunyaev-Zel’dovich effect;
- Studying globular clusters;
- Exploring the Tully-Fisher relationship;
- Investigating the Faber-Jackson relationship.
The primary observable components of galaxies [ edit edit code ]
The primary observable components of galaxies encompass [6] :
- Typical stars with diverse masses and ages, a portion of which are enclosed within clusters.
- Dense remnants of evolved stars.
- Cold gas-dust medium.
- The least dense hot gas with a temperature of 10 5 -10 6 K .
Double stars are not detected in nearby galaxies, but based on the proximity of the Sun, there should be a significant number of multiple stars. The gas-dust medium and stars are composed of atoms, and their collective is referred to as the baryonic matter of the galaxy. The non-baryonic matter includes the mass of dark matter and the mass of black holes [6] .
The speed of rotation of galaxies [ edit edit code ] .
The rotation speed of a galaxy refers to how quickly the different parts of the galaxy rotate around its center. This speed is the overall speed acquired through various processes. It’s important to note that the rotation speed of a galaxy is different from the circular velocity Vc, which is solely caused by the force of gravity and is defined as the necessary velocity for an object to move in a circle under the influence of gravity towards the center. In the general case, the rotation speed is also affected by the radial pressure gradient P of the interstellar gas.
In this context, Φ represents the gravitational potential, and ρg represents the gas density, with ▼ representing the nabla operator.
To estimate the rotation rate of different components in a galaxy, different methods are used. For gas, the rotation rate is determined by measuring the Doppler shift of emission lines. For stars, the rotation rate is determined by measuring the Doppler shift of absorption lines in their spectra. The procedure for determining the rotational velocity is as follows.
The observed velocity is the sum of the velocity of the galaxy as a whole and the velocity of internal motion. The velocity of the galaxy as a whole (V0) is usually assumed to be the velocity of the central region. In the case of distant galaxies, this velocity is primarily caused by the Hubble expansion of the Universe, while the intrinsic velocity is assumed to be negligible.
The velocity observed after accounting for the overall velocity of the galaxy is known as the line-of-sight velocity (Vr). To calculate the rotational velocity of the galaxy at a specific distance, we must consider the effects of projection. This requires knowledge of the inclination angle of the galaxy axis with respect to the line of sight (i) and the angle (φ) between the major axis of the galaxy and the line connecting the galaxy center and the observed point. Therefore, to convert Vr to Vφ, we need five parameters: the galaxy velocity V0, the angles i and φ, and the coordinates of the galaxy center relative to any point in the image.
where l represents the distance along the slit from the center of the galaxy. Nevertheless, the most comprehensive data regarding the movement within the galaxy can be obtained by analyzing the velocity field, which consists of a series of measurements of radial velocities taken at numerous points across the galaxy disk. To obtain the velocity field, two-dimensional spectroscopy techniques are employed, typically utilizing either a multichannel receiver or a Fabry-Perot interferometer. Additionally, radio observations of gas in H I lines offer a two-dimensional representation of the velocity distribution within the galaxy [7].
Mass and Dimensions [ edit edit code ].
Galactic entities lack distinct boundaries. It is impossible to precisely determine the point at which a galaxy ends and intergalactic space begins. For instance, while a galaxy may have a particular size in the optical spectrum, its radius, as determined by radio observations of interstellar gas, could be several times larger. The measured mass of a galaxy is also contingent on its dimensions. The dimensions of a galaxy are typically defined as the photometric dimensions of the 25th magnitude isophote within a square angular second in the B filter. The standard notation for these dimensions is D25 [8] .
The estimation of disk galaxies’ mass is based on the rotation curve within a specific model. The optimal model choice depends on both the shape of the rotation curve and the general concept of galaxy structure. To make rough estimates of elliptical galaxies’ mass, it is necessary to consider the dispersion of stellar velocities as a function of distance from the center and the radial density distribution [9].
The mass of cold gas in the galaxy can be determined by the intensity of the H I line. If the recorded radiation flux density from the galaxy or any of its parts is denoted as Fν, then the corresponding mass can be calculated using the following formula:
where D is the distance in megaparsecs and the flux is measured in Jans.
Estimating the mass of a molecular gas is a challenging task due to the absence of excited lines in cold gas for the most common molecule, H2. To overcome this obstacle, scientists rely on the intensity of spectral lines emitted by the CO molecule, denoted as ICO. The proportionality coefficient between the emission intensity of CO and its mass varies depending on the metallicity of the gas. However, the main source of uncertainty stems from the cloud’s low transparency. This causes the cloud to absorb most of the emitted light from its inner regions, resulting in only the light from the cloud’s surface reaching the observer [10].
The galaxy’s spectrum consists of the radiation emitted by its various objects. On average, a galaxy’s spectrum exhibits two peaks. The primary source of radiation is the stars, with the majority of their intensity falling within the optical range (the first peak). Galaxies typically contain a significant amount of dust, which absorbs optical radiation and re-emits it in the form of infrared radiation. This accounts for the second peak in the infrared range. If the luminosity in the optical range is considered as one, a relationship between the sources and types of radiation can be observed [11].
| Gamma | Active nuclei of certain galaxies; sources that produce single, brief bursts of gamma rays. | |
| X-ray | 10−3—10−4 | Accretion disks of binary star systems in close proximity; hot gas; active nuclei. |
| Optical | 1 | Stars with varying temperatures; circumstellar dust disks in the near-infrared; gas emissions. |
| Far infrared | 0.5—2 | Interstellar dust heated by starlight; active nuclei and dust in certain galaxies. |
| Radio | 10−2—10−4 | Synchrotron radiation; thermal emission from H II regions; H I emission lines. |
The enigma of the dark halo [ edit edit code ]
If all the mass of galaxies is contained within stars, the density of matter in the galaxy can be estimated from the brightness of the stellar population, assuming that the mass-luminosity ratio remains relatively constant throughout the galaxy. As the galaxy approaches its outer edge, the brightness of the stars decreases, resulting in a lower average density of stars and a corresponding decrease in their rotational speed. However, the observed rotation curves of galaxies paint a different picture: beyond a certain point, the rotational speeds of stars are unexpectedly high compared to the density predicted by the mass-luminosity ratio. We can explain this phenomenon by proposing that at large distances from the galaxy’s center, mass becomes the dominant factor, exerting its influence solely through gravitational interaction.
According to measurements of the satellite motions of large galaxies, it has been observed that the dark halo is significantly larger in size compared to the galaxy’s optical diameter.
Massive dark halos have been discovered in all kinds of galaxies, albeit in varying ratios with respect to the visible matter [12].
Morphology [ edit edit code ].
can be rephrased as
Morphological Analysis [ edit edit code ].
A junction extends from the inner tips of the spiral branches (blue) to the nucleus of the galaxy. NGC 1300
Nucleus – A minuscule region situated at the core of the galaxy. When discussing galactic nuclei, the focus is usually on the nuclei of galaxies, specifically active galactic nuclei, where phenomena cannot be accounted for by the characteristics of the stars concentrated within them.
A disk – A relatively thin layer where the majority of galaxy objects are concentrated. It is comprised of the gas-dust disk and the stellar disk.
The polar ring – A rare component. In the classical scenario, a galaxy with a polar ring possesses two disks rotating in perpendicular planes. In the classical case, the centers of these disks coincide. The cause of polar rings is not entirely understood [13].
The brightest inner part of the spheroidal component is known as the bulge.
The outer spheroidal component is called the halo. The demarcation between the bulge and halo is not clearly defined and is subjective.
A spiral branch, also known as a spiral arm, is a concentration of interstellar gas and primarily young stars in a spiral shape. It is believed that these spiral arms are formed by density waves influenced by various factors, although the exact origin is still a topic of ongoing research.
A bar – is a dense, elongated structure composed of stars and interstellar gas. According to calculations, it is the primary source of interstellar gas supply to the galaxy’s center. However, most theories assume that the thickness of the disk is much smaller than its dimensions, meaning that the disk is flat. Therefore, most models simplify the disk as two-dimensional, and there are very few calculations for three-dimensional disk models. Only one three-dimensional calculation of a galaxy with a bar and gas exists in the literature [14]. According to the author, the gas does not reach the galaxy’s center but passes by at a considerable distance.
The main components of a galaxy are the gas-dust disk, the stellar disk, and the spheroidal component. There are four main types of galaxies [15]:
- Galaxies without a disk component or with a weakly contrasted one are called elliptical galaxies (E).
- Spiral galaxies (S) are characterized by the presence of spiral branches, which can sometimes degenerate into rings.
- Lenticular galaxies (S0) have a similar structure to spiral galaxies, except for the absence of a clear spiral pattern. This is due to the low content of interstellar gas and the resulting low rate of star formation.
- Irregular galaxies (Irr) have an irregular wispy structure and typically contain a significant amount of interstellar gas, making up to 50% of the galaxy’s mass.
| Spherical component | Entire galaxy | Yes | Very dim | |
| Stellar disk | None or faint | Yes | Main component | |
| Gas-dust disk | No | No or very sparse | Yes | |
| Spiral arms | No or only near the nucleus | No or weakly expressed | Yes | No |
| Active nuclei | Occur | No | ||
| Percentage of total galaxy count | 20 % | 55 % | 5 % |
It is often found that the somewhat more elaborate Hubble classification of galaxies into subtypes is quite useful. The Hubble division (or the Hubble Cameron), which includes all galaxies, is based on their visually observed structure. Although it accurately describes elliptical galaxies, the classification of the same spiral galaxy can vary.
In 2003, Michael Drinkwater, a researcher from the University of Queensland, made a groundbreaking discovery of a previously unknown type of galaxy known as ultracompact dwarf galaxies [16].
- A galactic year refers to the length of time it takes for the Solar System to complete one revolution around the center of our Galaxy. The exact duration of this time period is uncertain due to factors such as the speed of our system and the distance to the Galaxy’s center. Estimates of the Galactic year range from 225 to 250 million Earth years.
According to contemporary theories, the genesis of the Solar System commenced approximately 4.6 billion years ago through the gravitational collapse of a fraction of a colossal interstellar molecular cloud. The majority of the matter gravitated towards the center of the collapse, ultimately leading to the birth of a star known as the Sun. The remaining matter that did not succumb to the gravitational pull coalesced into a rotating protoplanetary disk encircling the Sun, which later gave rise to the planets, their satellites, asteroids, and other celestial bodies within the solar system.
The Milky Way (also referred to as our Galaxy or simply Galaxy with a capital G) is a spiral galaxy encompassing the Earth, the Solar System, and all discernible individual stars visible to the unaided eye. It belongs to the category of spiral galaxies with a central bulge.
An intergalactic star, also referred to as a rogue star, is a star that is not gravitationally bound to any galaxy. The scientific community has been actively discussing the origins of these stars since their discovery in the late 1990s. The main factors believed to contribute to their formation include galaxy collisions or the passage of a binary star near a supermassive black hole.
This compilation gathers potential candidates for Earth-like exoplanets that are located within a maximum distance of 50 light-years from the solar system. The planets are arranged in ascending order based on their distance from the corresponding parent star.
A galaxy supergroup is a collection of numerous galaxy groups and galaxy clusters that exist within the larger structure of the Universe.
Mentions in literature
The movement of stars and nebulae within the Galaxy is quite intricate. Firstly, they are involved in the rotation of the Galaxy around an axis that is perpendicular to its equatorial plane. This rotation is not uniform, as different parts of the Galaxy have varying rotation periods. For instance, the Sun and the surrounding stars in a vast region spanning several hundred light-years complete a full revolution in approximately 200 million years. Given that the Sun, along with its planetary family, has been in existence for around 5 billion years, it has made about 25 revolutions around the Galaxy’s axis of rotation during its evolution from a gas nebula to its current state. We can thus state that the Sun’s age is equivalent to 25 “galactic years,” in other words, the age of flourishing…
Related concepts (continued)
The solar system is a collection of planets and other celestial objects that revolve around the Sun. It originated from the gravitational collapse of a gas and dust cloud approximately 4.57 billion years ago.
A planetary alignment is a fascinating astronomical event where multiple planets in the solar system align on the same side of the Sun within a specific region of space. During this alignment, the planets are relatively close to each other in the sky.
A planet, derived from the Greek word “πλανήτης” (planētēs), refers to a heavenly body that orbits a star or its remnants. It is large enough to have a spherical shape due to its own gravity, but not massive enough to sustain nuclear fusion. Additionally, a planet must have cleared its orbit of smaller objects called planetesimals.
Earth’s doppelganger, also known as Earth analog or Twin Earth, is a theoretical exoplanet that shares many similarities with our own planet. It is located within the habitable zone of a star and has a comparable size, mass, and temperature to Earth.
The existence of a ninth planet in the outer region of our solar system is still a subject of debate. This hypothetical planet’s gravitational pull could potentially explain certain irregularities in the orbital distribution of objects beyond Neptune’s orbit, particularly those found in the diffuse disk outside the Kuiper belt. The undiscovered planet is believed to be similar in size to Neptune, with a mass equivalent to ten Earths and a diameter two to four times larger than Earth’s. It is thought to have an elongated orbit with a period of approximately 15,000 Earth years. However, the search for this planet is still ongoing.
Mini-Earth is a planet that is considerably smaller in mass compared to Earth and Venus. In our solar system, this category of planet includes Mars and Mercury. Detecting planets of this kind using the radial velocity method is almost impossible due to their low mass, so the transit method is currently the most effective. Despite the challenges, one of the first mini-Earths outside our solar system was discovered near the millisecond pulsar PSR 1257+12. The smallest mini-Earth found so far is WD 1145+017 b, which has a radius of only 0.15.
A double planet is an astronomical term used to describe a binary system consisting of two celestial objects that meet the definition of a planet and have enough mass to exert a gravitational influence greater than that of the star they orbit.
A solar day is defined as the period of time it takes for a celestial body to complete one full rotation around its axis in relation to the center of the Sun. More precisely, it is the time interval between two instances of the Sun reaching its highest or lowest point in the sky (known as culminations) as observed from a specific location on Earth (or any other celestial body).
Following the discovery of Neptune in 1846, there was speculation about the possible existence of another planet beyond its orbit. The search for this hypothetical planet began in the mid-19th century. In the early 20th century, Percival Lowell dedicated his efforts to locating this “Planet X”. He proposed the idea of Planet X to explain the discrepancies between the predicted and observed orbits of gas giants like Uranus and Neptune, suggesting that these deviations were due to the gravitational influence of an unseen ninth planet.
Interstellar space refers to the area of space that is bordered by the orbit of the furthest planet from its star.
There are three classifications of stars that are similar to the Sun: solar-type stars, solar-analog stars, and solar doubles. Studying these stars is crucial for gaining a better understanding of the Sun’s characteristics and determining whether it is unique or typical among other stars. It also provides insights into the potential existence of habitable planets around other solar-type stars.
An astronomical transit, also known as a passage, occurs when one celestial body passes in front of another, partially obscuring it. This phenomenon is observed from a specific vantage point and has significant astronomical implications.
The conditional region in space known as the habitable zone (HZ) in astronomy is defined based on the assumption that the conditions on the surface of planets within this zone will be similar to those on Earth, allowing for the presence of liquid water. Consequently, these planets (or their moons) are considered favorable for the development of life similar to that found on Earth. The likelihood of life originating is highest within the habitable zone, which is typically located in close proximity to a star (known as the circumstellar habitable zone).
Super-Earth, on the other hand, refers to a class of planets with a mass greater than that of Earth but significantly less than that of gas giants. The term “super-Earth” solely describes a planet’s mass and is not dependent on its proximity to its star or any other criteria.
In the field of astrobiology and planetary astrophysics, the Galactic habitable zone refers to the area within the galaxy that offers the most favorable conditions for the emergence and sustainable development of life. Specifically, this concept encompasses a range of factors (such as the metallicity of stars and the frequency of catastrophic events like supernova explosions) that provide a certain level of probability in determining whether a specific region in the galaxy is suitable for the formation of planets similar to Earth.
Galactica, derived from the Greek word “γᾰλαξίας” meaning “Milky Way” from “γάλα, γάλακτος” which translates to “milk,” refers to a gravitational system that consists of stars, star clusters, interstellar gas and dust, dark matter, and planets. Every object within a galaxy contributes to its motion relative to a central point of mass.
Mercury, the smallest of the Earth-group planets, is the planet closest to the Sun in the Solar System. It is named after the ancient Roman god of commerce, Mercury the Fast, because it moves through the celestial sphere at a faster pace than other planets.
The Great Attractor, also known as the Great Center of Attraction, is a gravitational anomaly that exists in intergalactic space. It is located approximately 75 Mpc away from Earth, which is equivalent to about 250 million light-years. This massive object, with a mass of approximately 5⋅1016 Mʘ (or 105 times the mass of the Milky Way), is believed to be the result of a massive collision between galaxies. Despite its immense size, the average density of matter in the vicinity of the Great Attractor is not significantly higher than the average density of the Universe.
The outer planets, also known as upper planets, are planets in the Solar System that orbit beyond the observer’s location. In the case of an observer on Earth, these planets are located beyond Earth’s orbit, starting with Mars.
The Mars hoax, also known as the Martian hoax, was a hoax that gained popularity in 2003 through email distribution. It falsely claimed that on August 27, 2003, Mars would appear larger than the full Moon to the naked eye. This hoax continued to circulate annually on social media and other platforms from 2005 to 2016. However, it was based on a misinterpretation of the original report about the distance between Mars in 2003.
The giant planets, which include Jupiter, Saturn, Uranus, and Neptune, are positioned outside the asteroid belt in the solar system. These outer planets share several physical characteristics and are commonly referred to as such.
Tidal acceleration is the result of the gravitational-tidal interaction within a natural satellite-centered body system. This phenomenon leads to changes in the satellite’s orbit and the central body’s rotation around its axis, as exemplified in the Earth-Moon system. Additionally, tidal acceleration can cause heating within a planet’s interior, as seen in Io and Europa, and likely played a significant role in the ancient Earth.
Galaxies known as disc galaxies possess a stellar or gas-star disk that appears flattened by a circular accumulation of stars. It is possible for these galaxies to feature a central ellipsoidal area called the Bulge, although this is not always the case. Disc galaxies are the most prevalent type of galaxy, accounting for over two-thirds of the observed stellar systems. This percentage does not include dwarf galaxies, which are typically more abundant but smaller in size. Typically, the majority of a galaxy’s mass is contained within its galactic disk.
A planetary system is a collection of stars and various astronomical objects that are not stars themselves. These objects include planets and their satellites, dwarf planets and their satellites, asteroids, meteoroids, comets, and cosmic dust. They all orbit around a common center of mass. A stellar system, on the other hand, consists of multiple gravitationally bound stars with closed orbits and their own planetary systems. The solar system, which includes the Earth and orbits around the Sun, is an example of a planetary system.
A runaway star is a star that moves at an unusually high speed compared to the surrounding interstellar medium. The proper motion of such a star is often measured relative to the stellar association it was once a part of before being ejected from it. Our Sun is just one of the 400 billion stars in our galaxy, the Milky Way. The galaxy has a slow rotation, taking 250 million years to complete one revolution. The majority of stars in the galaxy are part of stellar systems with their own planetary systems.
An orphan planet, also known as a vagrant planet, planemo, planet stranger, interstellar planet, free-floating planet, free-flying planet, quasi-planet, or solitary planet, is a celestial object with a size and shape similar to that of a planet. However, unlike a typical planet, an orphan planet is not held in orbit by the gravitational pull of a star, brown dwarf, or even another planet (although it may have moons). In the case that an orphan planet is located within a galaxy, it does not have a specific star or system that it is gravitationally bound to.
Stellar kinematics is a branch of astronomy that examines the movement of stars in space. This field encompasses the study of star velocities within the Milky Way and its neighboring galaxies, as well as the internal motion of distant galaxies. By analyzing the kinematic properties of stars in various components of the Milky Way, such as the thin disk, thick disk, bulge, and stellar halo, scientists can gain valuable insights into the Galaxy’s formation and evolution. Kinematic data plays a crucial role in this research.
The celestial dome at night is adorned with a plethora of celestial bodies, primarily comprised of stars. The unaided human eye has the ability to discern stars with a magnitude of 5-6. When the sky is clear and free from clouds, one can behold the awe-inspiring sight of up to 800 stars with a magnitude of 5th, and an astonishing 2,500 stars with a magnitude of 6th. The majority of these stars are situated in close proximity to the ethereal Milky Way, which, in itself, contains an incomprehensible number of stars within our very own galaxy.
Eryda, also known as Xena, is the second largest dwarf planet in our Solar System, right after Pluto. It is the most massive and is located farthest from the Sun. Eryda belongs to the category of trans-Neptunian objects called plutoids. Prior to the XXVI Assembly of the International Astronomical Union, Eryda was considered the tenth planet. However, on August 24, 2006, the International Astronomical Union established a new definition for classical planets, which Eryda and Pluto do not meet.
The issue of evaluating the stability of the solar system has been a longstanding qualitative problem in celestial mechanics. In the context of Newtonian gravitation theory, a system of two bodies is considered stable. However, in a system of three bodies, motion can occur, potentially resulting in the expulsion of one of the bodies from the system. Furthermore, the planets in the solar system have finite sizes and could collide with each other when they come into close proximity. Present analysis suggests that the solar system is likely stable in terms of planet ejection, but.
Stellar days refer to the duration of a celestial body’s rotation around its own axis in an inertial frame of reference, typically the frame associated with distant stars. In the case of Earth, it represents the time taken for one complete revolution of the Earth around its axis relative to distant stars.
Space velocities (denoted as v1, v2, v3, and v4) are critical velocities that characterize the motion of space objects within the gravitational fields of celestial bodies and their systems. These velocities are used to describe the type of motion exhibited by spacecraft within the realm of celestial bodies, including the Sun, Earth, Moon, other planets, their natural satellites, as well as asteroids and comets.
Neptune, the eighth planet in our Solar System, is located at the farthest distance from Earth. It is also the fourth largest planet in terms of diameter and the third largest in terms of mass. In fact, Neptune has a mass 17.2 times greater than Earth and an equatorial diameter 3.9 times larger than Earth’s.
The Atons, a collection of asteroids that orbit near Earth, have paths that intersect with Earth’s orbit from the inner side. These asteroids are located further from the Sun at their farthest point (aphelion) than Earth is at its closest point (perihelion), with a distance of Q > 0.983 astronomical units. However, their major semi-axis is even smaller than Earth’s. Read more: List of multiple planetary systems
In astronomy, conjunction refers to the alignment of celestial bodies when their ecliptic longitudes are equal. It is also sometimes referred to as the direct ascent of bodies, rather than their ecliptic longitude. During a conjunction, two bodies are relatively close to each other on the celestial sphere, although the moment of conjunction does not always coincide with the moment of maximum approach. The term conjunction is also used in astrology.
As the capital city continues to celebrate the 55th anniversary of the first human space flight, a special exhibition called “Russian Space” will open on May 18. In preparation for this event, we have gathered a collection of intriguing facts about the universe. These questions, which may seem basic, are often asked by children but can sometimes leave adults stumped. What is the temperature like in space? Can we hear the sound of planets? And just how many stars are there in the universe? Find out all the answers in our informative article.
Galaxies visible to the naked eye from Earth
When observing from Earth, it is possible to see up to four galaxies without the aid of a telescope. In the Northern Hemisphere, both our Milky Way and the Andromeda Galaxy (M31) are visible. In the Southern Hemisphere, the Large and Small Magellanic Clouds can be seen.
Among these galaxies, the Andromeda Galaxy stands out as the largest and closest to us. However, with the use of a sufficiently powerful telescope, it is possible to observe thousands of additional galaxies. These galaxies will appear as indistinct patches of various shapes.
When we gaze at the night sky, we are peering into the past.
As we lift our eyes to the celestial expanse above and catch sight of the familiar stars, we are actually witnessing events that occurred many years ago.
This is due to the fact that the light we see from these stars has traveled a great distance before reaching our eyes. The stars visible from Earth are located light years away from us, and the farther a star is, the longer it takes for its light to reach us.
For instance, the Andromeda galaxy is situated 2.3 million light years away. That is precisely how long it takes for its light to reach us. When we observe the galaxy, we are perceiving it as it appeared 2.3 million years ago. And even our own Sun’s light arrives eight minutes later than the present moment.
Interestingly, our Sun rotates at different speeds depending on its location. It completes one rotation in 25.05 Earth days at the equator and 34.3 days at the poles.
Space is not completely silent
Although we perceive sound through the vibrations of air, the absence of air in space means that we cannot hear any sounds.
However, this does not mean that there is complete silence. In reality, even in a rarefied gas or vacuum, there can be long wavelength sounds that are beyond the range of our hearing. These sounds can be produced by collisions of gas and dust clouds or by supernovae explosions.
Naturally, we cannot directly hear these electromagnetic waves. However, certain spacecraft are equipped with instruments that can capture radio radiation, which can then be converted into sound waves by scientists. For example, we can now listen to the “voice” of the giant Jupiter, as captured by the Cassini spacecraft in 2001.
Actually, the concept of temperature as we commonly understand it doesn’t really apply to the vastness of outer space. Temperature is a property of matter, and it is virtually nonexistent in the vacuum of space.
However, outer space is far from being devoid of activity. It is literally filled with radiation coming from various sources, such as collisions between gas-dust clouds or supernovae, among others.
It is theorized that the temperature in outer space approaches absolute zero, which is the lowest possible temperature that a physical object in the Universe can reach. Absolute zero serves as the foundation for the Kelvin scale, equivalent to minus 273.15 degrees Celsius.
The temperature of space is influenced by various celestial bodies such as planets, satellites, asteroids, meteorites, comets, and cosmic dust. These factors contribute to temperature fluctuations in space. Additionally, the vacuum of space acts as a natural insulator, similar to a giant thermos. Without the presence of an atmosphere, objects in space can heat up rapidly.
For instance, placing a body in space near Earth and exposing it to sunlight can cause the temperature to reach up to 473 Kelvin, which is approximately 200 degrees Celsius. Therefore, space can exhibit both hot and cold temperatures, depending on the specific location of measurement.
Space isn’t just black
While we perceive the night sky as black and the daytime sky as blue, this is due to the Earth’s atmosphere. It may seem straightforward: space appears black because it lacks light. However, what about the stars? In reality, there are countless stars in the cosmos, saturating it with their luminosity.
From our vantage point on Earth, we don’t see stars everywhere because the light from many of them fails to reach us. Additionally, our solar system resides in a relatively tranquil, rather uneventful, and dim region of the galaxy. Moreover, the stars in our vicinity are widely dispersed. The closest star to our planet, Proxima Centauri, is located 4.22 light years away. That’s 270,000 times farther than the distance between Earth and the Sun.
Indeed, if we take into account the entirety of the electromagnetic radiation spectrum, space is predominantly filled with radio waves emitted by various celestial bodies. If our visual perception was capable of detecting these waves, our experience of the Universe would be significantly brighter. However, at present, our perception is confined to what appears to be complete darkness.
The largest known star in the Universe
Undoubtedly, we are referring to the most colossal star within our knowledge. Scientists have estimated that there are over 100 billion galaxies in the Universe, each of which contains several million to hundreds of billions of stars. It is not difficult to conceive that among these stars, there may exist colossal entities that we are not yet aware of.
It has been discovered that the issue of identifying the largest star is not clear-cut even among scientists themselves. As a result, we will present information about three currently recognized behemoths. For an extended period, VY in the Big Dog constellation held the title of the largest star. Its radius ranges from 1300 to 1540 times that of the Sun, and its diameter measures approximately two billion kilometers. To put this into perspective, the Sun has a diameter of 1.392 million kilometers. If we were to envision our star as a one-centimeter sphere, VY’s diameter would be a staggering 21 meters.
The largest star known to us is R136a1, located in the Large Magellanic Cloud. It’s truly mind-boggling, but this star weighs as much as 256 Suns combined. Additionally, it holds the title for being the brightest star in existence. This blue hypergiant outshines our own star by a staggering factor of ten million. However, when it comes to sheer size, R136a1 is not the biggest star out there. Despite its incredible luminosity, it is located a daunting 165 thousand light years away from Earth, making it impossible to see with the naked eye.
Currently, the reigning champion in terms of size is the red hypergiant NML Swan. Scientists estimate that this star has a radius of 1650 times that of our own sun. To put this into perspective, let’s imagine that we replaced our sun with NML Swan at the center of our solar system. This gigantic star would occupy the entire expanse of space within the orbit of Jupiter.
The orbit of the Earth serves as a repository for space debris. Over 370 thousand objects ranging in weight from a few grams to 15 tons orbit our planet.
Most of the planets in the solar system are visible to the naked eye.
During certain times, we can observe Mercury, Venus, Mars, Jupiter, and Saturn from Earth. These planets have been known since ancient times.
Distant Uranus can also be seen without a telescope from time to time. However, before its discovery, the planet appeared as a faint star. The existence of Uranus, Neptune, and Pluto, due to their considerable distance, was only revealed by scientists with the aid of a telescope. The only planets we cannot see with the naked eye from Earth are Neptune and Pluto, the latter of which is no longer classified as a planet.
Is there life beyond Earth?
There is another celestial body within our solar system that some scientists believe could potentially support life, even if only in its most primitive forms. This celestial body is Saturn’s moon, Titan.
Titan is unique in that it has a significant number of lakes. However, these lakes are not like those on Earth, as they are filled with liquid methane and ethane.
Despite these differences, Titan is often compared to Earth during its early stages of development. This has led some scientists to speculate that there may be simple life forms existing in the underground reservoirs of Saturn’s moon.
- Space debris refers to failed spacecraft, spent rockets, and other devices, as well as their resulting debris, that are located in orbits near Earth.
- Weightlessness is a condition where the gravitational forces acting on a body do not exert pressure on its parts.
- Solar wind is a continuous stream of high-velocity electrons and protons emitted by the Sun.
- A black hole is a region of space with an incredibly strong gravitational field that prevents matter and radiation from escaping. They form during the final stages of evolution of extremely large stars.
- Exoplanets are planets that exist outside of our solar system.
- A comet is a small object that orbits the Sun in a highly elongated elliptical path. As it gets closer to the Sun, it forms a tail or cloud of dust and gas.
- A galaxy is a system of stars and star clusters, interstellar gas, dust, and dark matter that are held together by gravity.
- A star is a massive ball of gas that emits light and is held together by its own gravity and internal pressure.
- A rocket is a flying vehicle that moves by expelling part of its own mass, resulting in jet thrust. It can fly without the need for air or gas medium.
- A spaceport is an area with a complex of special facilities and technical systems specifically designed for launching spacecraft.
- Gravity is the force of attraction between material objects.
- A planet is a celestial body that orbits a star. It is large enough to be rounded by its own gravity but not large enough to undergo thermonuclear reactions.
- An asteroid is a relatively small celestial body in the Solar System that orbits the Sun. It is much smaller and less massive than planets and has an irregular shape and no atmosphere.
