The word “galaxy” (ancient Greek. Γαλαξίας) is derived from the Greek name of our galaxy (κύκλος γαλαξίας means “milky ring” – a description of the observed phenomena in the night sky).  When astronomers suggested that the various celestial objects, considered spiral nebulae can be huge cluster of stars, these objects were called “island universes” or “star island”. But later, when it became clear that these objects are similar to our galaxy, the two terms are no longer used and has been replaced by the term “galaxy”.
Galaxy (ancient Greek. Γαλαξίας – milky, milky) – huge, gravitationally bound system of stars and star clusters, interstellar gas and dust, and dark matter. All objects in the galaxy are involved in the movement of the common center of mass .
Galaxy – an extremely distant astronomical objects.
The nearest of these is generally measured in Mpc, and to far – in terms of the redshift z. It is because of the distance to distinguish with the naked eye in the sky can be just three: the Andromeda Galaxy (visible in the northern hemisphere), the Large and Small Magellanic Clouds (visible in the south). Allow images of other galaxies to individual stars could not be until the beginning of XX century. By the beginning of the 1990s there were no more than 30 galaxies, in which it was possible to see individual stars, and all of them were part of the local group. After the launch of a space telescope “Hubble” and the commissioning of a 10-meter ground-based telescopes, the number of galaxies, in which it was possible to distinguish individual stars has increased dramatically.
Galaxies are very diverse: among them there is a sphere-like elliptical galaxies, disc spiral galaxies, barred galaxies (bar), dwarf, irregular, etc. As for the numerical values, then, for example, their weight varies from 107 to 1012 solar masses, for comparison – the mass of our Milky Way galaxy is 2.1011 solar masses. Diameter of galaxies – from 5 to 250 kpc (16-800 thousand light-years), for comparison – the diameter of our galaxy about 30 kiloparsecs (100,000 light years). The largest known in 2012 galaxy IC 1101 has a diameter of more than 600 kpc.
One of the unsolved problems in the structure of galaxies is dark matter, which manifests itself only in the gravitational interaction. It can be up to 90% of the total mass of the galaxy, and may be entirely absent, as in some dwarf galaxies .
In the space of galaxies are distributed unevenly in the same area you can find a group of nearby galaxies, and can not find any, even the smallest galaxies (called voids). The exact number of galaxies in the observable universe is unknown, but appears to them about a hundred billion (1011).
Distance from the observer to the galaxy as a physical characteristic is not included in any process that occurs with the galaxy. The need for information on the distance to the galaxy occurs when: identifying neglected events, such as gamma-ray bursts, the study of the universe as a whole, the study of the evolution of the galaxies themselves, determining the mass of the galaxies and their sizes, etc.
All more or less modelenezavisimye methods for determining distances to galaxies can be divided into two types: the measurement facility in the galaxy, the distance to which a negligible amount is different from the distance to the galaxy, and the redshift.
The first way – photometric method, using the so-called standard candles, the luminosity of which is considered to be known. The distance can be calculated by the following formula:where m – the apparent magnitude, M – the absolute magnitude, and R – the distance measured in parsecs. At the present stage, as these standard candles are used :
Cepheid, knowing that the pulsation period, you can see their luminosity. The first subject on which measured the distance to other galaxies.
SNe Ia. It was with them in the 90-ies of XX century opened the accelerated expansion of the universe.
The second method is based on the empirical Hubble law and more dependent on the model than the previous one.
where H0 – Hubble constant. If we take now common ΛCDM-model (with the same Hubble constant), it will be any significant difference in the z ~ 10, which allows it to be ranked as relatively modelenezavisimym.
There are also some highly modelezavisimyh ways :
on the Sunyaev – Zel’dovich
for globular clusters,
Depending on the Tully – Fisher
Depending on the Faber – Jackson.
The main components of the observed galaxies include :
Normal stars of different masses and ages, some of which is contained in clusters.
Compact remnants evolved star.
Cold gas-dust environment.
Most rarefied hot gas with a temperature of 105-106 K.
Binary stars in nearby galaxies are not observed, but judging by the solar neighborhood, multiple stars should be enough. Gas-dust environment and the stars are made of atoms, and their collection is called baryonic matter galaxy. From non-baryonic dark matter include the mass and the mass of black holes.
The speed of rotation of galaxies
Under the rotation speed of the galaxy means the speed of rotation of the various components of the galaxy around its center. This rate – is the total rate, acquired in the course of various processes. The speed of rotation of the galaxy should be distinguished from the circular velocity , which is caused only by the force of gravity and is, by definition, the required speed of a body moving in a circle under the force of gravity to the center. The speed of rotation, in general, is also due to the radial pressure gradient P interstellar gas.
For different components of the galaxy rotation speed is estimated differently. For gas – the Doppler shift of the emission lines. For the stars – by the Doppler shift of the absorption lines of stars.
Weight and size
Galaxies do not have clear boundaries. It is impossible to say exactly where it ends and begins galaxy intergalactic space. For example, if in the optical galaxy is one size, defined by the radius of the radio observations of the interstellar gas of the galaxy may be ten times greater. Depends on the size and the measured mass of the galaxy. Usually, the size of the galaxy size understand photometric isophotes 25th magnitude with a square arcsecond in the filter B. Standard notation of this size – D25 .
Mass disk galaxies measured by the rotation curve within a certain model. Choosing the optimal model of the galaxy rests both on the shape of the rotation and the common understanding of the structure of the galaxy. For rough estimates of the masses of elliptical galaxies is necessary to know the velocity dispersion of stars in relation to the distance from the center, and radial density distribution.
An evaluation of the molecular gas is very complex, because the spectrum of the most common molecule H2 has no lines excited in the cold gas. Therefore, the original data is the intensity of the spectral lines of the molecule CO (ICO). The coefficient of proportionality between the intensity of CO and its mass depends on the metallicity of the gas. The biggest uncertainty is associated with maloprozrachnostyu cloud over her main fraction of the light emitted by the inner regions, is absorbed by a cloud, so the observer light comes only from the surface of the clouds.
The spectrum of the galaxy
Spectrum of galaxies consists of radiation of all of its constituent objects. Range of the average galaxy has two local maxima. The main source of radiation – the stars, the maximum intensity of most of them is in the optical range (first two weeks). Usually a lot of dust in the galaxy, which absorbs radiation in the optical range and re-emits it in the infrared. Hence the second maximum – in the infrared region. If the luminosity in the optical range to the unit, then there is the following relationship between the sources and types of radiation .
If the entire mass of the galaxy lies in the stars, then knowing the mass-luminosity relation, and assuming that it does not change much with radius, the density of matter in the galaxy can be estimated from the brightness of the stellar population. Closer to the edge of the galaxy to its dull, then the average density of stars falling, and with it must fall and the speed of rotation of the stars. However, the observed rotation curves of galaxies suggest radically different picture: starting from a certain point the speed of rotation of stars are abnormally high for the density obtained from the dependence of the mass-luminosity relation. Explain the high velocity of the stars at the edge of the disc is possible, assuming that at large distances from the center of the galaxy mass plays a major role, manifesting itself only through the gravitational interaction.
Independently can come to the conclusion that there is dark matter, when measured on the basis of the total mass of the stellar disk stability conditions.
The measured velocity of satellites of massive galaxies suggest that the size of the dark halo is several times larger than the optical diameter of the galaxy.
The presence of massive dark halos were found in all types of galaxies, but in different proportions to the luminous matter.
Core – an extremely small area in the center of the galaxy. When it comes to the cores of galaxies, most often speak of active galactic nuclei, where the process can not explain the properties concentrated in these stars.
Disk – a relatively thin layer, which is concentrated most of the objects of the galaxy. Divided into gas-dust disk and a stellar disk.
Polar Ring – a rare component. In a classic case of a polar ring galaxy has two disks rotating in perpendicular planes. The centers of these discs in a classic case of the same. The cause of the polar ring is not completely clear.
Spheroidal component – the sphere-like distribution of stars.
Bulge (born bulge – bulge) – the bright interior of the spheroidal component.
Halo – external spheroidal component. The border between the bulge and halo blurred and rather conventional.
Spiral arms (Corrugated) – consolidation of interstellar gas and predominantly young stars in a spiral. Likely to be density waves, for various reasons, but the question of their origin is still not completely resolved.
Bar (jumper) – looks like a stretched tight formation consisting of stars and interstellar gas. According to calculations, the main supplier of the interstellar gas to the center of the galaxy. However, almost all theoretical constructs based on the fact that the thickness of the disk is much smaller than its size, in other words, the disc is flat and almost all models – simplified two-dimensional model calculations of three-dimensional models of drives very little. A three-dimensional calculation of the galaxy with a bar and gas in the known literature is only one . According to the author of this calculation, the gas does not get to the center of the galaxy, and goes pretty far.
Key features include gas-dust disk, stellar disk and spheroidal components. There are four main types of galaxies
Elliptical galaxies (E) – the galaxy in which the disk component is not present, or it is low contrast. All the rest of the galaxy disc.
Spiral galaxies (S) – galaxies have spiral arms. Branches can sometimes degenerate into a ring.
Lenticular galaxies (S0) – galaxies, their structure is not different from the spiral, except for the lack of a clear spiral pattern. The reason is low in the interstellar gas, and thus the low rate of star formation.
Irregular galaxies (Irr) – they are characterized by irregular patchy structure. As a rule, they have a lot of interstellar gas, and 50% of the mass of the galaxy.
Large-scale association of galaxies
In the images of galaxies shows that galaxies really lonely little (so-called field galaxies). About 95% of the galaxies form clusters of galaxies . In these, as in normal galaxies, suggests the presence of dark matter that makes up most of the mass of 10-30% – this intergalactic gas, and about 1% of the mass of the stars themselves.
The smallest in size and the most common in the universe cluster comprising several dozen galaxies, is a group of galaxies. They often dominated by a massive elliptical or spiral galaxy, which is due to tidal forces eventually destroys the satellite galaxies and increases its mass by absorbing them. In these clusters, the speed of recession of galaxies from each other due to the Hubble expansion of the Universe is dominated by weak and random peculiar velocities. From the analysis of these random velocities and virial can get a lot of these groups.
Cluster of galaxies called the union of several hundred galaxies, which can contain both individual galaxies and groups of galaxies. Usually, when viewed on the scale there are several very bright supermassive elliptical galaxies . These galaxies have a direct impact on the formation and the formation of the structure of the cluster.
Supercluster – the largest union type galaxies, includes thousands of galaxies. In the scale of superclusters of galaxies are arranged in strips and thread around the vast emptiness sparse. The form of such clusters can be different from the chain, such as Markarian’s chain, to the walls, as the Sloan Great Wall. In large-scale Universe appears isotropic and homogeneous .
Our galaxy is one of the Local Group of galaxies, dominating her with Andromeda. In the Local Group a diameter of about 1 Mpc are more than 40 galaxies. The Local Group itself is part of the Virgo Supercluster, the main role in which she plays the Virgo cluster, in which our galaxy is not included.
collision of galaxies
If the average distance between galaxies is no more than an order of magnitude greater than their diameter, the tidal effects become significant galaxies. The impact of each of these components of galaxies in different environments respond differently. If the distance is relatively large, but also great and the time of flight of the two galaxies relative to each other, the more massive galaxy can win a hot intergalactic gas surrounding the next galaxy, thus denying its source, to replenish the internal reserves of the interstellar gas, consumption for star formation .
If further reduce the distance, it is possible that the more massive component together with the intergalactic gas, chip away and the dark halo of the galaxy, leaving her with virtually no dark matter. Very often this happens when a large difference in the masses of the galaxies. Also, if the distance is not great, as the interaction time is small and, in any galaxy density waves of gas, which can cause a massive burst of star formation and the appearance of the spiral arms.
The limiting case of interaction – is merging galaxies. According to modern concepts, first merge the dark galactic halo. Then galaxies begin to approach each other in a spiral. And only then begin to merge stellar components, causing the surrounding gas density waves and bursts of star formation.
Orbiting telescope “Hubble” in 2006, photographed the interacting galaxies, two of which break on the third part of his acting on it by gravity (in the constellation of the Southern Fish, removed from the earth at a distance of 100 million light-years).
Galactic collisions are very common in the universe. An analysis of 21,902 galaxies (post early in 2009 ), it was found that almost all of them in the past met with other galaxies. Also confirmed the assumption that about 2 billion years ago the Milky Way collided with another galaxy.
Processes in active galactic nuclei
Galactic nuclei are signs of activity, if :
electromagnetic spectrum of the object is much broader than normal galaxies, sometimes extending from radio to hard gamma radiation;
there is a “change” – change the “power” of the source of radiation at the observation point (usually, this is a period of 10 minutes in X-rays up to 10 years in the optical and radio bands);
There are features of the emission spectrum, which can be seen on the movement of the hot gas at high speeds;
there is a visible morphological features, including emissions and “hot spots”;
There are features of the spectrum of radiation and its polarization, you can imagine including the presence of a magnetic field.
Active galactic nuclei are divided into Seyfert galaxies, quasars, BL Lac objects, radio galaxies.
According to modern concepts, active galactic nuclei due to the presence in their cores of supermassive black holes , which is the accretion of galactic gas. And the difference types of galaxies with active nuclei due to the difference in the angle of inclination of the plane of the galaxy relative to the observer.
The phenomenon of gravitational lensing
Walking around the massive body, the light beam is deflected. Thus, a massive body is able to collect a parallel beam of light in a certain focus, building an image. In addition, the increased brightness of the source due to the change of its angular size.
In 1937, Fritz Zwicky predicted the gravitational lensing of galaxies. Although not yet built a recognized model of this phenomenon for galaxies, now, this effect becomes important in terms of observational astronomy. It is used for:
ΛCDM-test model of the universe
search for dark matter in clusters of galaxies
search for distant galaxies.
At this point in the database NED More than 700 lensed galaxies and quasars.
Dark matter in galaxy clusters
Watching the velocity dispersion of galaxies in clusters, F. Zwicky with S. Smith discovered that obtained from virial mass is much greater than the total mass of galaxies . It has been suggested that within clusters of galaxies, as in the galaxy, there is a kind of dark matter, which manifests itself only by the gravitational way.
Refute or confirm it is possible, knowing the gravitational potential at each point, and based on Newton’s law of universal gravitation. The gravitational potential can be found by examining the effect of gravitational lensing. Based on these data, scientists have come to two conclusions. On the one hand, it was confirmed the existence of dark matter. On the other, it was discovered the unusual behavior of the gas and dark matter. It used to be that in all the processes of dark matter should drag the gas (this assumption underlies the hierarchical evolution of galaxies). However, MACSJ0025.4-1222, is a collision of two massive clusters of galaxies, the behavior of gas and dark matter are diametrically opposed.
Search for distant galaxies
Search for distant galaxies involves the following problems:
receiver sensitivity in the infrared region, where due to cosmological redshift moves all visible light up to the line Lα (Lyman-alpha) and the Lyman jump, much worse;
radiation from distant galaxies as weakened because of cosmological effects, and because of the fact that young galaxies in the modern view, at high z (which means that at the earlier stages of their lives) is much smaller than the Milky Way, and are similar to the Clouds of Magellan.
Multiple beam amplification of light caused by the gravitational lensing, helping to solve both problems by allowing the observation of galaxies at z> 7. Based on these theoretical concepts, the team of astronomers has observed massive clusters. As a result of their observations, a list of candidate sites sverhdalekie galaxy .
Star – a large-scale process in the galaxy, in which the mass of interstellar gas begin to form stars. Spiral arms, the overall structure of the galaxy stellar population, luminosity and chemical composition of the interstellar medium – the results of this process. The size of the area covered by the star formation is generally less than 100 pc. However, there are complex starburst called superassociations, size comparable to the irregular galaxy.
In our and several nearby galaxies may direct observation of the process. In this case, signs of star formation is occurring :
presence of stars of spectral types OBA and related facilities (areas HII, outbreaks of new and supernovae);
infrared radiation, as from hot dust and young stars from themselves;
radio emission from gas and dust disks around newborn stars emerging and;
Doppler splitting of molecular lines in a rotating disk around the stars;
Doppler splitting of molecular lines thin fast jets (jet), escaping from these drives (the poles) with a speed of about 100 km / s;
existence of associations, clusters and stellar complexes with massive stars (massive stars are born almost always in large groups);
the presence of globules.
As the distance decreases, and the apparent angular size of the object, and, from a certain moment, to make out the individual objects within the galaxy is not possible. Then the criteria flowing in distant galaxies star formation are :
luminosity in the emission lines, in particular Hα;
increased power in the UV and blue part of the spectrum, which is directly responsible for the emission of massive stars;
increased emission at wavelengths near 8 microns (infrared range)
Increased capacity of the thermal and synchrotron radiation in the radio;
high power X-ray emission associated with the hot gas.
In general, the process of star formation can be divided into several stages: the formation of large gas complexes (with a mass of 107 Mʘ), the appearance in them gravitationally bound molecular clouds, the gravitational attraction of the densest parts of them before the stars, gas heating radiation of young stars and the outbreak of new and supernovae , care of the gas.
Most often, the star forming region can be found :
in the cores of large galaxies
at the ends of the spiral arms,
on the periphery of irregular galaxies,
in the most vibrant part of the dwarf galaxy.
The star is a self-regulating process: after the formation of massive stars and their short life is a series of powerful flares, sealing and heating gas. On the one hand, the seal leads to faster compression relatively dense clouds inside the complex, but on the other side of the hot gas starts to leave the star forming region, and the more it is heated, the faster it goes.
Milky Way, also called simply the Galaxy, is a large barred spiral galaxy with a diameter of about 30 kpc (or 100,000 light years) and a thickness of 1000 light-years (up to 3000 in the area of the bulge). The sun to the solar system are within the galactic disk, filled with dust and absorbing light. Therefore, in the sky, we see the band of stars, but ragged, resembling bunches of milk. Due to the absorption of light as the Milky Way galaxy is not fully understood: the rotation curve is not constructed, not completely understood morphological type, unknown number of spirals, etc. The galaxy contains about 3.1011 stars , and its total weight is about 3.1012 solar masses.
Important role in the study of the Milky Way plays study star clusters – a relatively small gravitationally bound objects containing from hundreds to hundreds of thousands of stars. Their gravitational coupling is probably due to their origin. Therefore, based on the theory of stellar evolution, and knowing the location of the cluster stars in the Hertzsprung – Russell, you can calculate the age of the cluster. Clusters scattered and divided into ball.
Ball – old star clusters with spherical shape, concentrated toward the center of the galaxy. Globular clusters may have an age of more than 12 billion years.
Scattered – a relatively young clusters are aged up to 2 billion years old, some even going star formation processes. The brightest stars of open clusters – the young stars of spectral type B or A, and in the youngest clusters still have blue supergiants (Class O).
Due to the small (relative to the cosmological scale) sizes, star clusters can only be observed directly in the Galaxy and its nearest neighbors.
Another type of object that is available only to watch around the sun – double stars. Significance for the study of binary stars of different processes in the galaxy, because thanks to them may determine the mass of the star, it is they can study the processes of accretion. New and SNe Ia – it is also the result of the interaction of stars in binary systems, called close binary systems.
History of the study of galaxies
In 1610, Galileo Galilei in the study of the Milky Way with a telescope discovered that the Milky Way is made up of a huge number of faint stars. In his treatise in 1755, based on the works of Thomas Wright (born Thomas Wright), Immanuel Kant suggested that the galaxy may be a rotating body, which consists of a huge number of stars held by gravitational forces akin to those that operate in the solar system, but a large scale. From the observation point on the inside of the galaxy (in particular, in our solar system), the resulting disc will be visible in the night sky as a bright band. Kant expressed the assumption that some of the nebulae visible in the night sky might be separate galaxies.
By the end of XVIII century, Charles Messier compiled a catalog containing the 109 brightest nebulae. Since the publication of the catalog until 1924 continued debate about the nature of these nebulae.
William Herschel suggested that the nebula may be a distant star system similar to the Milky Way. In 1785, he tried to determine the shape and size of the Milky Way and the position of the sun in it, using the “scoops” – counting stars in different directions. In 1795, observing the planetary nebula NGC 1514, it is clearly seen in its center a single star surrounded by misty substance. The existence of real nebulae, thus, could not be questioned, and it was not necessary to think that all misty spots – distant systems.
In the XIX century it was believed that the stars unresolvable nebulae are emerging planetary systems. But NGC 1514 was an example of the late stage of evolution of the primary nebula which is condensed central star.
By the middle of XIX century, John Herschel, son of William Herschel, discovered 5,000 more nebulous objects. Built on the basis of their distribution was the main argument against the assumption that they are distant “island universes” like our Milky Way. It was found that there is a “zone of avoidance” – an area in which there is little or no such nebulae. This zone is located near the plane of the Milky Way and has been interpreted as a link to the nebulae of the Milky Way. The absorption of light, the most powerful in the galactic plane, it was still not known.
After the construction of his telescope in 1845 by Lord Ross was able to see the differences between elliptical and spiral nebulae. In some of these nebulae, he was able to isolate and separate light sources.
Rotation around the core of the galaxy predicted Marian Kowalski , who in 1860 “Scientific Notes of Kazan University,” published an article with its mathematical justification, and the edition was translated into French.
In 1865, William Huggins (born William Huggins) first received spectrum nebulae. The nature of the emission lines of the Orion Nebula is clearly talking about her gas composition, but the spectrum of the Andromeda galaxy (M31 Messier Catalogue) was continuous as the stars. Huggins concluded that this type of spectrum M31 caused by high density and opacity of its constituent gas.
In 1890, Agnes Clerk (born Agnes Mary Clerke) in a book on the development of astronomy in the XIX century, wrote: “The question is whether the nebula external galaxies, hardly worth discussing now. Research progress answered it. It’s safe to say that no competent thinker in the face of the evidence would not say that at least one of the nebula can be star systems comparable in size to the Milky Way “.
At the beginning of XX century Vesto Slipher (born Vesto Melvin Slipher) explained the spectrum of the Andromeda nebula reflects the light of the central star (for which he received the core of the galaxy.) This conclusion was based on photographs taken by James Keeler on 36-inch reflector. It was found 120,000 light fog. Range where it can be obtained, was reflective. As we know now, it was the reflection spectra (mainly dust) nebulae around stars of the Pleiades.
In 1910, George Ritchie (born George Willis Ritchey) on the 60-inch telescope at Mount Wilson Observatory got pictures in which it was evident that the spiral arms of large nebulae strewn starlike objects, but many of the images were blurred and misty. It could be a compact nebulae and star clusters, and several merged images of stars.
In 1912-1913 was open relationship “period – luminosity” for Cepheids.
In 1918, Ernst Opik to determine the distance to the Andromeda galaxy, and found that it can not be part of the Milky Way. Although it has received the value of 0.6 from the current value, it became clear that the Milky Way is not the entire universe.
In 1920, there was “The Great Debate” between Harlow Shapley and Heber Curtis. The central issue was to measure the distance to the Magellanic Clouds Cepheids and estimating the size of the Milky Way. Using an improved version of the method of buckets, Curtis concluded that small (diameter 15 kpc) flattened galaxy with the Sun close to the center. And also a short distance to the Magellanic Clouds. Shapley, based on estimates of globular clusters, gave a very different picture – a flat disk with a diameter of about 70 kpc from the Sun, located far from the center. The distance to the Magellanic Clouds were the same order. The outcome of the dispute was the conclusion about the necessity of another independent measurement.
In 1924, the 100-inch telescope, Edwin Hubble found in Andromeda 36 Cepheids and measured the distance to it, it was great (though 3 times less than today’s value). This confirmed that the Andromeda Galaxy – not part of the Milky Way. The existence of galaxies has been proven, and the “Great Debate” ended.
The modern picture of our galaxy came in 1930, when Robert Julius Tryumpler (born Robert Julius Trumpler) measured the effect of the absorption of light by studying the distribution of open clusters concentrated in the galactic plane.
In 1936 he built the Hubble classification of galaxies, which is still used today and is called the Hubble sequence.
In 1944, Hendrik van de Hulst (Hendrik van de Hulst) predicted the existence of radio waves with a wavelength of 21 cm, emitted by interstellar atomic hydrogen, which was discovered in 1951. This radiation is not absorbed by dust, allowed to further explore the galaxy due to the Doppler shift. These observations led to the creation of a model with a bridge in the center of the galaxy. Subsequently allowed to track the progress of radio telescopes, hydrogen and other galaxies. In the 1970s, it became clear that the total visible mass of galaxies (consisting of the mass of stars and interstellar gas), does not explain the speed of the gas. This has led to the existence of dark matter .
In the late 1940’s. AA Kalinyak, VI Krasovskii and VB Nikonov got the first image of the galactic center in the infrared range of the spectrum .
New observations made in the early 1990s by the space telescope “Hubble”, showed that the dark matter in our galaxy can not consist only of a very weak and small stars. He also obtained images of deep sky, dubbed Hubble Deep Field and the Hubble Ultra Deep Field, showing evidence that in our universe, there are hundreds of billions of galaxies .