The galaxies are huge sets of stars and clouds of gases and dust. They
are the bricks that form the Universe, and the Universe contains billions
of stars.
A galaxy is an actual universe itself: is is an auto gravitating
system, which generally evolves separately from the other galaxies.
Actually, often two or more galaxies interact,
they get close to each other undergoing a deformation due to gravitational
attraction, and they can even collide, causing very violent phenomena.
Only in the last decades scientists could understand what galaxies actually
are. When the observation instruments were not as powerful as the modern
ones, in fact, they appeared as small luminous regions with a nebulous,
not resolved appearance, present in all directions in the sky. Up to the
beginning of the twenties it was thought that the so-called "spiral nebulae"
were objects that belonged to our galaxy, the size of which were not exactly
known.
In 1920 it was discovered that the explosive stars called novae
actually belonged to two categories: the actual novae
and the supernovae, much more luminous.
This discovery was very important, because it became clear that the nova
observed in the Andromeda nebula in 1985 was actually a supernova. The
fact that it appeared as luminous as the novae of our galaxy indicated
that its distance was much bigger: therefore, the Andromeda nebula is external
to the Milky Way.
It was only in 1924 that the astronomer Edwin Hubble, with the Mount
Wilson telescope, succeeded in resolving some regions of the Andromeda
nebula, thus confirming that it is a real galaxy, external to our galaxy.
Andromeda is one of the nearest galaxies to the Milky Way: it is "only"
two million light years away from us.
The galaxies have very different shape, size and mass. There are gigantic
galaxies, that contains up to 10,000 billion stars, and dwarf galaxies
that contain some hundred thousand stars. The spiral galaxies have mean
radii that measure approximately 70,000 light years, but one of the largest
galaxies of this kind, NGC 1961, has a diameter that measures 300,000 light
years and a mass that is 2,000 billion times that of the Sun.
It is easy to find even larger galaxies among the elliptic ones, with
the size of over 300,000 light years and a mass that reaches up to 10,000
billion times that of the Sun; the dwarf galaxies belong to this morphological
type; they measure only 5,000 light years and their mass is only a few
million times that of the Sun.
The galaxies have different shape and characteristics and they are classified
in three large categories: spiral galaxies, elliptic galaxies and
irregular galaxies.
The spiral galaxies have the shape of a
disc that rotates around its own axis, with spiral arms that depart from
a central nucleus and envelop it; they are rich in gas and the arms contain
young stars, while the nucleus contains older stars. New stars are still
originating in the spiral galaxies, in a more or less intense way.
The elliptic galaxies have the shape of
an ellipsoid more or less flattened,
are poor in gas and have almost only old stars. No new stars are formed
in this type of galaxy, since there is no more gas available.
The irregular galaxies do not have a well
defined shape, they are rich in gas, dust and young stars. The star formation
is less intense in this type of galaxy.
The so-called "lenticular galaxies" or SO are part of the classification
too; it is a type of galaxy that is half way between a spiral and an elliptic.
The SO galaxies have a flat shape, like a disc, but without the arms that
characterize the spiral galaxies.
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The M85 SO type lenticular galaxy, in the Coma Berenicis constellation. (SEDS) |
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The NGC 5866 SO type galaxy, in the Draco constellation. (University of Arizona Astronomy Club) |
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The M100 galaxy is one of the most brilliant of the Virgo cluster. It is a spiral galaxy, like the Milky Way, that can be seen almost frontally, and its distance from us is some million light years. (HST) |
The galaxies were formed soon after the birth of the Universe, that
is approximately 15 billion years ago. In the beginning they were huge
clouds of gas, mainly Hydrogen, with a certain percentage of Helium.
These clouds underwent a gravitational instability which caused their
fragmentation and collapse, thus forming the stars
In the case of elliptic galaxies, many stars were formed almost
at the same time, and very little gas was left for the formation
of other stars. These galaxies remained therefore "frozen" in the form
they had at the beginning of their evolution.
In the spiral galaxies, on the contrary, the stars were formed more
slowly, and some gas remained available. The gas and the stars began to
rotate faster and faster, assuming the flat shape of a disc, while the
star formation continued gradually.
The main components of the galaxies are the stars. The coexistence of
young and old stars, which originated at different times, with frequencies
and modalities that differ from one galaxy to the other, is possible within
the same galaxy.
The stars can be isolated, or gathered in sets called
clusters. The clusters contain more
or less contemporary stars, that originated from the same cloud of gas.
They are divided in two categories, open and globular clusters. The open
clusters are sets of a few hundred or thousand stars, they have an irregular
shape and contain young and massive stars. When these evolve, after ten
or a hundred million years, the cluster disintegrates, because the gravitational
attraction of the stars that compose it is not strong enough to keep them
together.
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This image was obtained by overlaying the ultraviolet, visible and infrared light pictures. It shows in detail two star clusters in the Large Magellanic Cloud, one of the galaxies nearest to ours. These clusters are 166,000 light years far from us. (HST) |
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The G1 globular cluster, that contains at least 300,000 old stars. It orbits around the Andromeda galaxy (M31), the large spiral galaxy at 2 million light years from the Milky Way. G1 is the brightest globular cluster among all the galaxies of the Local Group, a group that includes the Milky Way and about twenty more galaxies. (HST) |
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The M15 globular cluster in Pegasus.
(HST) |
A galaxy is also constituted by a set of gas and clouds and ions that
is called interstellar matter.
After the stars, the most important component of a spiral galaxy is
gas; mainly Hydrogen, with a minor percentage of Helium, and traces of
other gases such as carbon oxide (CO), methane (CH4),
ammonia (NH3) and aqueous vapour (H2O).
The gas is aggregated in clouds, often very large, and it can be in
different states: there are clouds of neutral Hydrogen in the (H2)
molecular state, very cold and dense; clouds of neutral Hydrogen in the
atomic state, a little more rarefied, that are called H1 regions;
clouds of ionized Hydrogen, hot and rarefied, that surround the young and
massive stars and that are called HII regions.
The molecular clouds have a density of 103-104
grams per cm3 and a temperature of 10 degrees above absolute
zero, that is -263 degrees centigrade! In these clouds the new stars
are formed, in fact the gas is dense enough to collapse in response to
a gravitational perturbation. Such gas contains only a few molecules besides
H2 Hydrogen: carbon oxide (CO), the cyanogen radical (CN),
the methylidine radical (CH), the oxydryl radical (OH), water
(H2O), formaldehyde (H2CO), ammonia (NH3),
etc.
In the HI regions, Hydrogen has a temperature of approximately 100-300
Kelvin, that is less than 30 oC, and the density is approximately
100 particles/cm3
Neutral hydrogen is a very important component of spiral and irregular
galaxies, since it is present almost everywhere and it can be revealed
at long distances. At such densities and temperatures, in fact, Hydrogen
emits a spectral line with a wavelength of 21 cm, that is in the band of
radio waves. The radiation of such wavelengths is not disturbed by the
terrestrial atmosphere or by the clouds of interstellar gas, therefore
the line at 21 cm allows to reveal the presence of HI even at long distances,
and, through its redshift it
is possible to study its motion. The rotation motions of many spiral galaxies
were reconstructed with this method.
The HII regions are zones with ionized gas that surround the young
and massive stars. These stars are formed initially in clouds of neutral
gas, but when the nuclear reactions are triggered, their temperature
rises and the radiation that they emit becomes energetic enough to deprive
the gas atoms of their electrons, and the gas reaches the temperature of
10,000 degrees, approximately. When the star, during its evolution, cools
down, the gas too gets colder and the electrons recombine with the ions.
This gas can be recognized even at long distances, thanks to the fact
that some elements present in low quantities together with Hydrogen (Oxygen,
Nitrogen, Sulphur, etc...) emit very intense and characteristic spectral
lines, that are visible from other galaxies.
Grains of dust can also be present, mixed to the interstellar gas; mainly
silicates, graphite and other carbonaceous materials. The grains formed
by condensation of the chemical elements heavier than Helium; these are
synthesized within the stars during the nuclear fusion reactions, and then
released in space by the stellar wind or
by phenomena such as the explosion of supernova
Dark matter seems to be a very important component of many galaxies:
the nature of this matter is still unknown, it fills the entire Universe.
It does not emit nor absorb radiation, and therefore it can only be detected
through its gravitational effects on visible matter, such as the stars
and gas. The analysis of the dynamics of many galaxies, and therefore of
the gravitational field to which they are subject, revealed some anomalies.
Such anomalies can only be explained by assuming that these galaxies are
surrounded by huge massive halos of dark matter. Even the galaxies in the
clusters seem bound together by huge quantities of
dark matter.
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The clouds of gas and dust contained in the disc of our galaxy darken part of the extragalactic sky. The galaxies that are hidden by these clouds can be revealed by the emission, by the neutral Hydrogen that they contain, of a line in the radio band, with a wavelength of 21 cm. Such wavelength can penetrate through the clouds and reach us. The Dwingeloo 1 galaxy, that you can see in the picture, was discovered in this way. (HST) |
The distance scales in astrophysics
The astronomers deduce the distances of the celestial objects by using
direct methods (for the nearest stars of our galaxy), or indirect methods,
by using distance indicators. The distance indicators are
celestial bodies with particular properties, that allow to deduce their
distance quite exactly.
For example, a very powerful indicator is the supernova:
if you observe a supernova in a distant galaxy, the distance of the galaxy
can be deduced by the distance of the supernova.
One of the fundamental discoveries of modern cosmology is that all galaxies are moving away from each other with a relative speed (V) that increases with the distance. Such fact is expressed by the Hubble's law
V = Ho d
where d is the distance of the galaxy and Ho is Hubble's constant. A reliable method for measuring the distance of the galaxy from us is extremely important for the calculation of Hubble's constant, while their speed can be deduced from the redshift of the spectrum.
Each distance indicator must be calibrated by using more elementary indicators, before it can be used. The simplest distance indicators are the young and luminous stars around the Solar System. Being near to us, the distances of such stars can be deduced by using geometrical methods such as the parallax. From their apparent luminosity and their distance it is possible to deduce the luminosity or absolute magnitude. If it is possible to ascribe the same magnitude to all the stars of a certain spectral type, when stars of the same spectral type at a longer distance are observed, the distance can be deduced from the apparent and the absolute magnitude. In practise, these stars act as "standard candles".
Other primary distance indicators are the Cefeids
variable stars
These stars have periodic luminosity variations due to the pulsation
of their outer layers; it has been discovered that the pulsation period
increases with the luminosity of the "quiet" star. From the measure
of the pulsation period of a variable star it is possible to deduce its
absolute magnitude and then, from its apparent magnitude, the distance
can be deduced.
The Cefeids are used to deduce the distances of nearby galaxies, since
they are very luminous giant stars, visible at the distance of a dozen
million light years or more.
Novae and supernovae
are secondary distance indicators, since they are stars that explode reaching
very high luminosities. They are used to measure the distances of
galaxies that are so far that only the most luminous objects can be distinguished.
Novae and supernovae have constant characteristics in their light curves
(that is the trend of luminosity in time). For example, all the supernovae
of a certain type have the same absolute magnitude when they reach the
peak of their brightness: by measuring their apparent magnitude, we can
deduce their distance.
All the structures of the Universe tend to gather following a hierarchy:
the planets in a planetary system, the stars in clusters, the clusters
in galaxies. Likewise, the galaxies tend to gather in groups
with a dozen of members; the groups then gather in clusters of
galaxies that, together with other clusters, form the superclusters.
Our galaxy is part of the Local Group, one of the poorest groups of
galaxies; two only galaxies dominate the Local Group, ours and M31 (the
galaxy of Andromeda), that together form approximately 80% of the mass
of the system. The other galaxies are smaller, like the M33 spiral, or
even satellites of the larger ones; for example, the Small and the Large
Magellanic Cloud are two small satellite galaxies of the Milky way, at
approximately 180,000 light years from us. The local group has a diameter
of approximately 5-6 light years.
The clusters of galaxies have been known since the thirties; the first
was discovered in the Coma Berenicis constellation, and it is called Coma
cluster: it is situated at a distance of 350 million light years, and it
comprises a thousand galaxies. The richest cluster is that of the Virgo,
in the homonymous constellation, that contains 2,500 galaxies, 50 million
light years far from us. The diameter of the largest clusters of galaxies
measures approximately 60 million light years, although it is not easy
to determine its limits, since the density of galaxies gradually decreases
outwards, and often a cluster mingles with another.
There are regular clusters, with a spheroidal shape, that contain essentially
elliptic galaxies, and irregular clusters, that resemble the open clusters
of stars and contain every kind of galaxies.
The galaxies of a cluster are immersed in a halo of very hot gas, that
emits radiation in the X ray band.
The galaxies have a motion within the cluster, and sometimes they
interact colliding with each other. Very often there is a giant
elliptic galaxy in the centre of such clusters; originally, this probably
was a normal size galaxy, but due to the high energy collisions with other
galaxies, it began to mingle with them until it "swallowed" them, a phenomenon
known as galactic cannibalism. The interaction processes among galaxies
are very violent and involve large quantities of energy. In many cases
the interaction of two galaxies leads to their fusion; besides, it causes
an increase of star formation in the galaxies that are subject to it, and
it could be involved in phenomena such as the active
galaxies.
It seems that the interacting galaxies had (and still have) a
fundamental role in the evolution of the Universe and in the large scale
modification of its properties, such as the number or the density of galaxies.
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The spiral M100 galaxy in a picture of Hubble Space Telescope, that shows in detail the spiral arms. M100 is part of the huge cluster of the Virgo, together with 2,500 galaxies approximately, and it is visible in the Coma Berenicis constellation. (HST) |
In the last video first the ultraviolet spectrum of M81 is shown,
(dominated by the stars with an age of approximately one million years),
and the red spectrum (dominated by the stars with an age of approximately
5 billion years).
