快猫短视频

DON鈥橳 mess with the Milky Way Galaxy. In 1994, astronomers discovered the
ruins of a dwarf galaxy that had strayed too close to the shores of our Galaxy
and was being ripped apart by its tide. This luckless object was our Galaxy鈥檚
latest victim, and its stars will soon be devoured by the Milky Way鈥檚 insatiable
appetite.

Our Galaxy鈥檚 immense power stems from its enormous size and mass. The Milky
Way is a giant galaxy, far larger, brighter, and more massive than most other
galaxies in the Universe. In addition to the Sun, it harbours hundreds of
billions of stars, which orbit the Galactic centre in the same way that the
planets of the Solar System circle the Sun. Every star visible to the unaided
eye is part of the Milky Way. But, for every star you see, there are 50 million
others that also belong to the galaxy we call home.

Astronomers observe the Milky Way not only to learn about our immediate
neighbourhood but also to understand other galaxies. Galaxies are the basic
building blocks of the Universe, just as atoms are of matter. But only in the
Milky Way can astronomers scrutinise the ages, locations, orbits, and chemical
compositions of the faint stars that constitute most of all galaxies, and
thereby decipher a galaxy鈥檚 origin and evolution.

The Milky Way is more than just the galaxy to which we owe our existence; it
is a touchstone for the entire Universe.

Stars and gas

Galactic geography

THE MILKY WAY is a spiral galaxy that to an external observer would
appear as a beautiful pinwheel in space. But, because we live inside it and see
stars from different regions superimposed on one another, it took astronomers
centuries to discern the Galaxy鈥檚 spiral shape and, even today, we do not know
exactly what the Milky Way looks like. The first spiral galaxy was seen in 1845,
when Lord Rosse in Ireland used a large telescope to examine an object named
M51. But only in 1951 did the American astronomer William Morgan and his
colleagues map out regions of hydrogen gas that are ionised by bright blue stars
which populate spiral arms and thus prove that our own Galaxy was also a
spiral.

Although the spiral arms outshine the regions in-between, they do not
contain more stars. Instead, they harbour clouds of gas and dust that give birth
to new stars. A few of these new stars are bright, blue and massive. Massive
stars die quickly, before they drift out of the spiral arm that gave birth to
them; so nearly all massive stars reside in spiral arms. These stars鈥� brilliance
lights the arms and makes them look bright, beautiful and obvious.

The Sun lies in the Orion arm of the Milky Way, as do all the bright stars,
and most of the faint ones, that you see at night. The Orion arm gets its name
because it includes famous objects from that constellation, such as the Orion
Nebula and the stars Betelgeuse and Rigel. But stars from the Orion arm appear
in every other constellation as well. Of course, there are other arms to our
Galaxy. Lying about 6000 light years closer to the Galactic centre than the
Orion arm is the Sagittarius arm; lying about 6000 light years in the opposite
direction is the Perseus arm.

The spiral arms belong to the Galaxy鈥檚 disc, which is shaped like a thin
pancake and measures about 130 000 light years across. The Sun is about 27 000
light years from the Galaxy鈥檚 centre, or about 40 per cent of the way from the
centre to the edge. The disc鈥檚 main component is 2000 light years 鈥渢hick鈥�, and
the Sun lies near its midsection, which is called the Galactic plane. Stack two
CDs atop each other and you have a miniature model of just how thin the Galactic
disc is in relation to its diameter.

When we look up into this star-filled disc at night, we see a stronger band
of white light produced by the combined glow of innumerable stars. This band is
sometimes called the 鈥淢ilky Way鈥� because the Greeks and Romans thought it looked
like spilled milk. But every star visible to the naked eye, whether it is in
this band or not, is part of the Milky Way Galaxy. It is just like being in a
crowd of people. When you look into the crowd, you see lots of people. When you
look up or down, you see few or none.

In addition to stars, the disc contains interstellar gas and dust. This
material is extremely tenuous: on average the disc has only one atom per cubic
centimetre. In contrast, terrestrial air has 25 billion billion molecules per
cubic centimetre, so the interstellar medium would pass for a 鈥減erfect鈥� vacuum
on Earth. Yet our Galaxy is so huge that all of the interstellar gas and dust
adds up to several billion times more mass than our Sun (see Inside Science,
Numbers 45 and 70).

We owe our lives to interstellar matter, because 4.6 billion years ago it
gave birth to the Sun and the Earth. Today, clouds of interstellar gas and dust
continue to create new stars in the spiral arms. The Milky Way spawns about 10
stars a year, so the youngest stars in the Galaxy are younger than you or
me.

All the stars and gas orbit Sagittarius A* (pronounced 鈥渁y star鈥�), which is
probably a black hole at the Galaxy鈥檚 centre containing a million times more
mass than the Sun. Extending for thousands of light years in all directions from
the Galactic centre is the bulge, which appears in edge-on views of the Milky
Way as a bump at the Galaxy鈥檚 centre jutting above and below the otherwise
flattened disc (see Inside Science, Number 70). Recently, astronomers have found
that the bulge is somewhat elongated, in a direction roughly towards the Sun,
which means the Milky Way may be a barred spiral galaxy. If so, it confirms a
speculation made by the American astronomer G茅rard de Vaucouleurs in
1963.

Clues to the past

Galactic demographics

SURROUNDING the disc is the stellar halo, which contains some of the
Galaxy鈥檚 oldest stars, and surrounding that and everything else is the
mysterious dark halo, which emits little or no light but contains most of the
Galaxy鈥檚 mass. The evidence for the existence of the dark halo comes from the
high speed with which stars and gas in the outer disc revolve around the Galaxy,
as well as from the motions of the 10 galaxies that orbit ours (see Box). The
extent of the dark halo is uncertain, but its diameter is at least a quarter of
a million light years, and probably much greater, in which case the nearest
galaxies actually reside within it. The dark halo鈥檚 composition is unknown. It
could be made of dark stars, such as brown dwarfs or black holes, or it could be
composed of subatomic particles that we have yet to detect.

In 1943, the American astronomer Walter Baade formulated the concept of
stellar populations鈥擥alaxy-wide groups that contain billions of different
stars which nevertheless share similar properties. These properties, astronomers
now recognise, are age, location, how stars move and what they are made of.
Baade鈥檚 stellar population concept has allowed astronomers to decode the rich
historical record written in our Galaxy鈥檚 stars in order to paint a vivid
portrait of how the Milky Way formed and evolved.

Of the four properties that mark a stellar population, age is the most
crucial. The difference in age from one stellar population to another allows
astronomers to piece together the chronological sequence of the Galaxy鈥檚
evolution, much as a geologist reconstructs the Earth鈥檚 past by studying
different rock strata.

The second property of a stellar population, location, refers primarily to
how the stars distribute themselves around the Galactic plane. Stars in some
stellar populations cling tightly to the Galactic plane, while stars in others
shoot far above and below it.

The third property of a stellar population, kinematics, describes how stars
move around the Galaxy. Every star follows its own orbit around the Galactic
centre. For example, the Sun revolves around the Galaxy once every 230 million
years on a fairly circular orbit. But some orbits are so elliptical that stars
journey from the remote stellar halo to the Galaxy鈥檚 central region.

The fourth and final property of a stellar population, metallicity, is a
star鈥檚 abundance of elements heavier than hydrogen and helium, which are the
two most common elements. This name exists because astronomers consider all heavy
elements鈥攅ven oxygen and neon which normally appear as gases鈥攖o be
metals. The Sun is metal-rich, with 2 per cent of its mass composed of metals.
But metallicity varies greatly from stellar population to stellar population,
and some stars have only a fraction of our Sun鈥檚 metallicity. Metals include
such life-giving elements as carbon and oxygen.

Stellar populations

Family features

AS astronomers presently understand the Milky Way, every star falls into
one of four stellar populations. The brightest and most prominent is the thin
disc population, which includes the Sun and 96 per cent of the stars near it,
such as Alpha Centauri, Sirius, Vega, Betelgeuse, and Rigel. Stars in the thin
disc have a wide range of ages: some are newborn objects, others are
middle-aged, like the Sun, and others are older still, with ages of about 10
billion years. As the name implies, thin disc stars lie close to the Galactic
plane, usually within 1000 light years of it. Kinematically, the stars have
fairly circular orbits, and the metallicity is high鈥攊ndeed, high enough
for life to exist.

The second great stellar population in the Milky Way is the thick disc, which
makes up about 4 per cent of the nearby stars. The brightest likely member is
the beautiful spring star Arcturus. As the name indicates, the thick disc is a
more distended population, with a typical star lying about 3500 light years from
the Galactic plane. Thick disc stars are old鈥攁bout 10 billion years
old鈥攁nd have more elliptical orbits around the Galaxy. Their metallicity
is about 25 per cent of the Sun鈥檚.

The third stellar population is the stellar halo, rare but extremely
important stars that give great insight into the Galaxy鈥檚 origin, for they
formed when the Galaxy itself did. Halo stars account for only about one nearby
star in a thousand. They often lie at large distances above and below the
Galactic plane and have extremely elliptical orbits: in a single orbit, the
distance of a halo star from the Galactic centre may vary wildly from a couple
of thousand light years to 100 000. In comparison, the Sun鈥檚 distance varies
only from 27 000 to 30 000 light years. Halo stars have low metallicities,
usually between 1 and 10 per cent of that of the Sun, meaning that life in the
halo is unlikely.

The fourth stellar population is the Galaxy鈥檚 bulge, which surrounds the
Milky Way鈥檚 centre. It is old and generally has a high metallicity. Because of
its distance from Earth, it is the least explored stellar population in the
Galaxy.

By comparing the three stellar populations nearest to the Sun鈥攖he thin
disc, the thick disc and the halo鈥攚e see that there is an age-metallicity
relation. The oldest stars鈥攖hose in the halo鈥攈ave the lowest
metallicity; the somewhat younger, but still old, stars in the thick disc have a
higher metallicity; and the youngest stars, in the thin disc, have the greatest
metallicity. Since stars usually preserve the metallicity that they were born
with, this means that the Milky Way鈥檚 metallicity has increased over time.

Smooth or messy?

Origin of the Galaxy

THIS, in turn, implies that the Galaxy鈥檚 stars have created the heavy
elements by fusing lighter ones together. This creation of new elements is
called nucleosynthesis. When the stars die, these newly formed heavy elements
are ejected into the Galaxy, where they enrich clouds of interstellar gas and
dust. These clouds give birth to new stars that consequently inherit higher
metallicities than their predecessors. The heavy elements on Earth鈥攖he
oxygen we breathe, the calcium in our bones, the iron in our blood鈥攚ere
created billions of years ago, by stars that have long since died. In a sense,
then, we are the heirs of those ancient stars.

Having fathomed the basic geography and stellar content of the Milky Way,
astronomers can attempt to piece together the Galaxy鈥檚 origin and evolution. In
1962, Olin Eggen, Donald Lynden-Bell and Allan Sandage, who were working
together in Pasadena, California, proposed that the Galaxy had formed from a
single huge ball of low-metallicity gas that was rapidly collapsing. Some stars
were born during this collapse and acquired the highly elliptical orbits and low
metallicities of the surrounding, infalling gas鈥攖hese are the ancient
stars we see today in the stellar halo. The collapse of the Galaxy was so rapid,
said Eggen, Lynden-Bell and Sandage, that all halo stars formed within a very
short time of one another; about 200 million years鈥攍ess than 2 per cent of
the Galaxy鈥檚 total life.

Most of the infalling gas did not immediately condense into stars, however.
Instead, it fell into a swirling disc, where collisions among gas clouds had the
effect of making their orbits nearly circular. This gas had a higher
metallicity, because some of the halo stars had already enriched it with heavy
elements. As a result, the stars that formed in the disc had circular orbits and
higher metallicities than stars in the halo, thereby explaining the properties
of the Galaxy鈥檚 disc population.

The first serious challenge to the precepts of Eggen, Lynden-Bell and Sandage
came in 1978, when Leonard Searle and Robert Zinn, also working in Pasadena,
proposed that the Galaxy鈥檚 origin had been much more chaotic. The Galaxy鈥檚 outer
halo was not born from a single collapsing entity, said Searle and Zinn;
instead, it arose from the collision of numerous smaller galaxies, similar to
those that orbit the Milky Way today. In fact, the small galaxy mentioned
earlier that astronomers discovered in 1994鈥攚hich the Milky Way is
swallowing鈥攕hows the Searle and Zinn scenario in action.

During the 1980s, several astronomers who investigated the orbits and
metallicities of stars in the halo found evidence favouring the chaotic Searle
and Zinn scenario over the more elegant model of Eggen, Lynden-Bell and
Sandage.

However, more recent investigations of the kinematics and metallicities of
old stars have supported some aspects of the Eggen, Lynden-Bell and Sandage
theory, and today many believe that both pictures are necessary to fully
understand the Galaxy鈥檚 origin.

As astronomers continue to study the origin and evolution of the Galaxy, they
realise that the Milky Way is a giant that is bustling with hundreds of billions
of stars cloaked in an enormous halo of dark matter whose gravitational pull
anchors a surrounding empire of at least 10 lesser galaxies and asserts its
influence over other galaxies millions of light years away. More than just our
home galaxy, the Milky Way is one of the Universe鈥檚 most beautiful and
spectacular creations.

* * *

The vast Galactic empire

THE MILKY WAY is more than just a giant galaxy. It is also the hub of a
vast empire that stretches over a million light years and encompasses at least
10 other galaxies. These satellite galaxies orbit the Milky Way in the same
manner that the Moon circles the Earth.

The two biggest and brightest satellites are the Large Magellanic Cloud and
the Small Magellanic Cloud, which respectively lie 160 000 and 190 000 light
years from our Galaxy鈥檚 centre. Although the Magellanic Clouds are smaller than
the Milky Way, they outshine most other galaxies, for they contain billions of
stars.

At greater distances lie at least eight dwarf galaxies, which bear the names
of the constellations in which they lie. The first two dwarf satellites,
Sculptor and Fornax, were discovered in 1938. A typical dwarf contains just a
few million stars that are quite spread out. The faintest dwarfs emit less light
than the single brightest star in the Milky Way. The dimmest known galaxy in the
entire Universe is Draco, which is about 240 000 times brighter than the Sun.
For comparison, the Milky Way is 15 billion times brighter than the Sun.

The satellite galaxies reveal the Milky Way鈥檚 mass, because they orbit under
the influence of its gravity. The faster the satellites move, the more massive
our Galaxy must be, just as the planets of the Solar System would move faster if
the Sun had more mass. The motions of the satellite galaxies indicate the Milky
Way has roughly a million million times more mass than the Sun.

In part because of this huge mass, the satellite galaxies lead a dangerous
life, because our Galaxy鈥檚 tide tries to tear them apart. On Earth, the Moon
exerts a tide because one side of the Earth is closer to the Moon than the other
and so feels a greater gravitational force. Fortunately, the Earth is strong
enough to withstand the tide and is not torn apart. But the satellite galaxies,
and especially the dwarf satellites, are much more fragile. The stars of the
dwarfs are so spread out that they are only bound weakly to their home galaxy,
so it does not take much to pull the galaxies apart.

The Galactic tide depends on distance: halve a satellite鈥檚 distance from our
Galactic centre and the tide strengthens eightfold. Thus the nearest satellites
are in the most danger. In 1994, Rodrigo Ibata, Gerard Gilmore and Mike Irwin,
while working at the University of Cambridge, discovered the remains of a dwarf
galaxy just 60 000 light years from the Galactic centre鈥攃loser than any
other galaxy. This galaxy had been torn in two by the Milky Way鈥檚 tide.

Because they are so large, the Magellanic Clouds face an additional danger,
called dynamical friction. The Magellanic Clouds probably lie within the Milky
Way鈥檚 dark halo. As massive satellites such as these move through the sea of
objects constituting the dark halo, their gravity disturbs the dark matter,
creating a wake behind whose gravity slows down the satellites. Thus, the
satellites lose orbital energy, making them fall closer to the centre of the
Milky Way. This causes the Galactic tide on these satellites to strengthen, and
they split apart, splattering countless stars into the Milky Way鈥檚 halo. Within
10 billion years, the Magellanic Clouds will crash on the Galaxy鈥檚 shore, and
the Milky Way will feast on a banquet of billions of new stars that will only
augment its already mighty power.