The life of galaxies and the history of their study

2024 Author: Seth Attwood | [email protected]. Last modified: 2023-12-16 15:55

The history of the study of planets and stars is measured in millennia, the Sun, comets, asteroids and meteorites - in centuries. But galaxies, scattered throughout the Universe, clusters of stars, cosmic gas and dust particles, became the object of scientific research only in the 1920s.

Galaxies have been observed since time immemorial. A person with sharp eyesight can distinguish light spots in the night sky, similar to drops of milk. In the 10th century, the Persian astronomer Abd-al-Raman al-Sufi mentioned in his Book of Fixed Stars two similar spots, now known as the Large Magellanic Cloud and the galaxy M31, aka Andromeda.

With the advent of telescopes, astronomers have observed more and more of these objects, called nebulae. If the English astronomer Edmund Halley listed only six nebulae in 1716, then the catalog published in 1784 by the French naval astronomer Charles Messier already contained 110 - and among them four dozen real galaxies (including M31).

In 1802, William Herschel published a list of 2,500 nebulae, and his son John published a catalog of more than 5,000 nebulae in 1864.

Our closest neighbor, the Andromeda galaxy (M31), is one of the favorite celestial objects for amateur astronomical observations and photography.

The nature of these objects has long eluded understanding. In the middle of the 18th century, some discerning minds saw in them stellar systems similar to the Milky Way, but telescopes at that time did not provide an opportunity to test this hypothesis.

A century later, the opinion prevailed that each nebula is a gas cloud illuminated from the inside by a young star. Later, astronomers were convinced that some nebulae, including Andromeda, contain many stars, but for a long time it was not clear whether they are located in our Galaxy or beyond.

It was only in 1923-1924 that Edwin Hubble determined that the distance from Earth to Andromeda was at least three times the diameter of the Milky Way (in fact, about 20 times) and that M33, another nebula from the Messier catalog, was no less distant from us. distance. These results marked the beginning of a new scientific discipline - galactic astronomy.

In 1926, the famous American astronomer Edwin Powell Hubble proposed (and in 1936 modernized) his classification of galaxies by their morphology. Because of its characteristic shape, this classification is also called the "Hubble Tuning Fork".

On the "stem" of the tuning fork there are elliptical galaxies, on the prongs of the fork - lenticular galaxies without sleeves and spiral galaxies without a bar-bridge and with a bar. Galaxies that cannot be classified as one of the listed classes are called irregular, or irregular.

Dwarfs and giants

The universe is filled with galaxies of different sizes and masses. Their number is known very approximately. In 2004, the Hubble orbiting telescope discovered about 10,000 galaxies in three and a half months, scanning in the southern constellation Fornax a region of the sky that is a hundred times smaller than the area of the lunar disk.

If we assume that galaxies are distributed over the celestial sphere with the same density, it turns out that there are 200 billion in the observed space. However, this estimate is greatly underestimated, since the telescope was unable to notice a great many very faint galaxies.

Form and content

Galaxies also differ in morphology (that is, in shape). In general, they are divided into three main classes - disc-shaped, elliptical and irregular (irregular). This is a general classification, there are much more detailed ones.

Galaxies are not at all randomly distributed in outer space. Massive galaxies are often surrounded by small satellite galaxies. Both our Milky Way and neighboring Andromeda have at least 14 satellites, and most likely there are many more. Galaxies love to unite in pairs, triplets and larger groups of dozens of gravitationally bound partners.

The larger associations, galactic clusters, contain hundreds and thousands of galaxies (the first of such clusters was discovered by Messier). At times, a particularly bright giant galaxy is observed in the center of the cluster, which is believed to have arisen during the merging of smaller galaxies.

And finally, there are also superclusters, which include both galactic clusters and groups, and individual galaxies. Usually these are elongated structures up to hundreds of megaparsecs in length. They are separated by almost completely galaxy-free space voids of the same size.

Superclusters are no longer organized into any structures of a higher order and are scattered throughout the Cosmos in a random way. For this reason, on a scale of several hundred megaparsecs, our Universe is homogeneous and isotropic.

A disc-shaped galaxy is a stellar pancake revolving around an axis passing through its geometric center. Usually on both sides of the central zone of the pancake there is an oval bulge (from the English bulge). The bulge also rotates, but with a lower angular velocity than the disk. In the plane of the disk, spiral branches are often observed, abounding in relatively young bright luminaries. However, there are galactic discs without a spiral structure, where there are many fewer such stars.

The central zone of a disk-shaped galaxy can be cut by a stellar bar - a bar. The space inside the disk is filled with a gas and dust medium - the source material for new stars and planetary systems. The galaxy has two disks: stellar and gaseous.

They are surrounded by a galactic halo - a spherical cloud of rarefied hot gas and dark matter, which makes the main contribution to the total mass of the galaxy. The halo also contains individual old stars and globular star clusters (globular clusters) up to 13 billion years old. In the center of almost any disk-shaped galaxy, with or without a bulge, there is a supermassive black hole. The largest galaxies of this type contain 500 billion stars each.

Milky Way

The sun revolves around the center of a quite ordinary spiral galaxy, which includes 200-400 billion stars. Its diameter is approximately 28 kiloparsecs (just over 90 light years). The radius of the solar intragalactic orbit is 8.5 kiloparsecs (so that our star is displaced to the outer edge of the galactic disk), the time of a complete revolution around the center of the Galaxy is about 250 million years.

The bulge of the Milky Way is elliptical in shape and has a bar that was recently discovered. In the center of the bulge is a compact core filled with stars of various ages - from several million years to a billion and older. Inside the core, behind dense dusty clouds, lies a rather modest black hole by galactic standards - only 3.7 million solar masses.

Our Galaxy boasts a double stellar disk. The inner disk, which has no more than 500 parsecs vertically, accounts for 95% of the stars in the disk zone, including all young bright stars. It is surrounded by an outer disk 1,500 parsecs thick, where older stars live. The gaseous (more precisely, gas-dust) disk of the Milky Way is at least 3.5 kiloparsecs thick. The four spiral arms of the disk are regions of increased density of the gas-dust medium and contain most of the most massive stars.

The diameter of the Milky Way's halo is at least twice the diameter of the disk. About 150 globular clusters have been discovered there, and, most likely, about fifty more have not yet been discovered. The oldest clusters are over 13 billion years old. The halo is filled with dark matter with a lumpy structure.

Until recently, it was believed that the halo is almost spherical, however, according to the latest data, it can be significantly flattened. The total mass of the Galaxy can be up to 3 trillion solar masses, with dark matter accounting for 90-95%. The mass of stars in the Milky Way is estimated at 90-100 billion times the mass of the Sun.

An elliptical galaxy, as its name suggests, is ellipsoidal. It does not rotate as a whole and therefore does not have axial symmetry. Its stars, which mostly have a relatively low mass and considerable age, revolve around the galactic center in different planes and sometimes not individually, but in highly elongated chains.

New luminaries in elliptical galaxies rarely light up due to a shortage of raw materials - molecular hydrogen.

Like humans, galaxies are grouped together. Our Local Group includes the two largest galaxies in the vicinity of about 3 megaparsecs - the Milky Way and Andromeda (M31), the Triangulum galaxy, as well as their satellites - the Large and Small Magellanic Clouds, dwarf galaxies in Canis Major, Pegasus, Carina, Sextant, Phoenix, and many others - a total of about fifty. The local group, in turn, is a member of the local Virgo supercluster.

Both the largest and the smallest galaxies are of the elliptical type. The total share of its representatives in the galactic population of the Universe is only about 20%. These galaxies (with the possible exception of the smallest and faintest ones) also hide supermassive black holes in their central zones. Elliptical galaxies also have halos, but not as clear as those of disc-shaped ones.

All other galaxies are considered irregular. They contain a lot of dust and gas and are actively producing young stars. There are few such galaxies at moderate distances from the Milky Way, only 3%.

However, among objects with a large redshift, whose light was emitted no later than 3 billion years after the Big Bang, their share increases sharply. Apparently, all stellar systems of the first generation were small and had irregular outlines, and large disc-shaped and elliptical galaxies arose much later.

Birth of galaxies

Galaxies were born soon after stars. It is believed that the first luminaries flashed no later than 150 million years after the Big Bang. In January 2011, a team of astronomers processing information from the Hubble Space Telescope reported the probable observation of a galaxy whose light went into space 480 million years after the Big Bang.

In April, another research team discovered a galaxy that, in all likelihood, was already fully formed when the young universe was about 200 million years old.

The conditions for the birth of stars and galaxies arose long before it began. When the universe passed the 400,000 year mark, plasma in outer space was replaced by a mixture of neutral helium and hydrogen. This gas was still too hot to coalesce into the molecular clouds that give rise to stars.

However, it was adjacent to particles of dark matter, initially distributed in space not quite evenly - where it is a little denser, where it is more rarefied. They did not interact with the baryonic gas and therefore, under the action of mutual attraction, freely collapsed into zones of increased density.

According to model calculations, within a hundred million years after the Big Bang, clouds of dark matter the size of the current solar system formed in space. They combined into larger structures, despite the expansion of space. This is how the clusters of dark matter clouds arose, and then the clusters of these clusters. They sucked in space gas, allowing it to thicken and collapse.

In this way, the first supermassive stars appeared, which quickly exploded into supernovae and left behind black holes. These explosions enriched space with elements heavier than helium, which helped cool the collapsing gas clouds and therefore made possible the appearance of less massive second-generation stars.

Such stars could already exist for billions of years and therefore were able to form (again with the help of dark matter) gravitationally bound systems. This is how long-lived galaxies arose, including ours.

“Many of the details of galactogenesis are still hidden in the fog,” says John Kormendy. - In particular, this applies to the role of black holes. Their masses range from tens of thousands of solar masses to the current absolute record of 6.6 billion solar masses belonging to a black hole from the core of the elliptical galaxy M87, located 53.5 million light years from the Sun.

Holes in the centers of elliptical galaxies are usually surrounded by bulges made up of old stars. Spiral galaxies may have no bulges at all or have their flat similarities, pseudo-bulges. The mass of a black hole is usually three orders of magnitude less than the mass of the bulge - naturally, if it is present. This pattern is confirmed by observations covering holes with a mass from a million to a billion solar masses."

According to Professor Kormendy, galactic black holes gain mass in two ways. The hole, surrounded by a full-fledged bulge, grows due to the absorption of gas that comes to the bulge from the outer zone of the galaxy. During the merger of galaxies, the intensity of the influx of this gas increases sharply, which initiates outbursts of quasars.

As a result, bulges and holes evolve in parallel, which explains the correlation between their masses (however, other, as yet unknown mechanisms may work as well).

Researchers at the University of Pittsburgh, UC Irvine and Florida Atlantic University have modeled the collision situation between the Milky Way and the predecessor of the Sagittarius Dwarf Elliptical Galaxy (SagDEG) in Sagittarius.

They analyzed two options for collisions - with an easy (3x10¹⁰solar masses) and heavy (10¹¹ solar masses) SagDEG. The figure shows the results of 2.7 billion years of evolution of the Milky Way without interaction with a dwarf galaxy and with interaction with the light and heavy variant of SagDEG.

Bald-free galaxies and galaxies with pseudo-bulges are a different matter. The masses of their holes usually do not exceed 104-106 solar masses. According to Professor Kormendy, they are fed with gas due to random processes that occur near the hole, and do not extend across the entire galaxy. Such a hole grows regardless of the evolution of the galaxy or its pseudo-bulge, which explains the lack of correlation between their masses.

Growing galaxies

Galaxies can increase both in size and mass. "In the distant past, galaxies did this much more efficiently than in recent cosmological eras," explains Garth Illingworth, professor of astronomy and astrophysics at the University of California, Santa Cruz. - The rate of birth of new stars is estimated in terms of the annual production of a unit mass of stellar matter (in this capacity, the mass of the Sun) per unit volume of outer space (usually a cubic megaparsec).

At the time of the formation of the first galaxies, this figure was very small, and then began to grow rapidly, which continued until the Universe was 2 billion years old. For another 3 billion years, it was relatively constant, then began to decline almost in proportion to the time, and this decline continues to this day. So 7-8 billion years ago, the average rate of star formation was 10-20 times higher than the current one. Most observable galaxies were already fully formed in that distant epoch."

The figure shows the results of evolution at different times - the initial configuration (a), after 0, 9 (b), 1, 8 © and 2, 65 billion years (d). According to model calculations, the bar and spiral arms of the Milky Way could have formed as a result of collisions with SagDEG, which initially pulled at 50-100 billion solar masses.

Twice it passed through the disk of our Galaxy and lost some of its matter (both ordinary and dark), causing perturbations of its structure. The current mass of SagDEG does not exceed tens of millions of solar masses, and the next collision, which is expected no later than 100 million years later, will most likely be the last for it.

In general terms, this trend is understandable. Galaxies grow in two main ways. First, they obtain fresh starburst material by drawing in gas and dust particles from the surrounding space. For several billion years after the Big Bang, this mechanism worked properly simply because there was enough stellar raw material in space for everyone.

Then, when reserves were depleted, the rate of stellar birth dropped. However, galaxies have found the ability to increase it through collisions and mergers. True, for this option to be realized, the colliding galaxies must have a decent supply of interstellar hydrogen. For large elliptical galaxies, where it is practically gone, merging does not help, but in discoid and irregular galaxies it works.

Collision course

Let's see what happens when two approximately identical disk-type galaxies merge. Their stars almost never collide - the distances between them are too great. However, the gaseous disk of each galaxy is experiencing tidal forces due to the gravity of its neighbor. The baryonic matter of the disk loses part of the angular momentum and shifts to the center of the galaxy, where conditions for an explosive growth in the rate of star formation arise.

Some of this substance is absorbed by black holes, which also gain mass. In the final phase of the unification of galaxies, black holes merge, and the stellar disks of both galaxies lose their former structure and are dispersed in space. As a result, one elliptical is formed from a pair of spiral galaxies. But this is by no means the complete picture. Radiation from young bright stars can blow some of the hydrogen out of the newborn galaxy.

At the same time, the active accretion of gas onto the black hole forces the latter from time to time to shoot jets of enormous energy particles into space, heating gas throughout the galaxy and thus preventing the formation of new stars. The galaxy is gradually quieting - most likely forever.

Galaxies of different sizes collide differently. A large galaxy is capable of swallowing a dwarf galaxy (at once or in several steps) and at the same time preserving its own structure. This galactic cannibalism can also stimulate star formation.

The dwarf galaxy is completely destroyed, leaving behind chains of stars and jets of cosmic gas, which are observed both in our Galaxy and in neighboring Andromeda. If one of the colliding galaxies is not too superior to the other, even more interesting effects are possible.

Waiting for the super telescope

Galactic astronomy survived nearly a century. She started practically from scratch and achieved a lot. However, the number of unsolved problems is very large. Scientists are expecting a lot from the James Webb Infrared Orbiting Telescope, which was slated to launch in 2021.