20 Things You Didn't Know About Galaxies
1. Eighteenth-century philosopher
Immanuel Kant was one of the first people
to theorize that the Milky Way was not the
only galaxy in the universe. Kant coined the
term island universe to describe a galaxy.
2. Astronomers now estimate that there are
100 billion galaxies in the observable
universe.
3. One of the earliest uses of the English
term Milky Way was in Geoffrey Chaucer’s
14th-century poem “The House of Fame.”
He likened the galaxy to a celestial roadway.
4. While we’re talking road trips: Due to the
expansion of the universe, all other galaxies
are receding from our own. Galaxies farther
from the Milky Way are speeding away
faster than those nearby.
5. Some of the galaxies receding from the
Milky Way are ellipsoidal, like footballs.
Galaxies can also be thin and flat with
tentacle-like arms — just like the Milky Way.
6. Galaxies come in irregular shapes, too,
including many dwarf galaxies. These
galaxies, the smallest in the universe,
contain only a few hundred or a few
thousand stars (compared with 100 billion
stars in the Milky Way).
7. You’ll often find dwarf galaxies clustered
around larger galaxies.
8. Dwarf galaxies frequently lose their stars
to their larger neighbors via gravity. The
stars stream across the sky as the dwarf
galaxies are ripped apart. Alas, you can’t see
it with the naked eye.
9. You also can’t see the enormous black
hole lurking in the center of the Milky Way,
though if you’ve ever looked at the
constellation Sagittarius, the archer, you’ve
looked in the right direction.
10. Most galaxies have a black hole at the
center, and astronomers have found the
mass is consistently about 1/1000th the
mass of the host galaxy.
11. Two of the closest galaxies to the Milky
Way — the Small Magellanic Cloud and the
Large Magellanic Cloud — may not have
black holes. Or, because both are low-mass
galaxies, their central black holes may be
too small to detect.
12. Every galaxy does have dust, though.
Produced by stars, the dust causes light to
look redder than it really is when observed
visually, which can make it difficult for
astronomers studying properties of stars.
13. That dust can really travel, too. Some
galaxies drive galactic winds, expelling dust
and gas at hundreds of kilometers per
second into the intergalactic medium, the
space between galaxies.
14. These winds are caused by starlight
exerting pressure on the dust and gas; the
fastest galactic winds are in distant galaxies
that are forming stars more rapidly than the
Milky Way.
15. The Milky Way rotates at about 250
kilometers per second (about 560,000
mph) and completes a full revolution about
every 200 million years.
16. One galactic revolution ago, dinosaurs
ruled the Earth.
17. Galaxies rotate faster than predicted
based on the gravity of their stars alone.
Astronomers infer that the extra
gravitational force is coming from dark
matter, which does not emit or reflect light.
18. Dark matter aside, galaxies are mostly
empty space. If the stars within galaxies
were shrunk to the size of oranges, they
would be separated by 4,800 kilometers
(3,000 miles).
19. If galaxies were shrunk to the size of
apples, neighboring galaxies would only be
a few meters apart. The relative proximity
of galaxies means that galaxies occasionally
merge.
20. In about 4 billion years, the Milky Way
will merge with the Andromeda galaxy. The
result of the merging process — which will
take at least a hundred million years — will
be an ellipsoidal galaxy nicknamed
“Milkomeda.”
Immanuel Kant was one of the first people
to theorize that the Milky Way was not the
only galaxy in the universe. Kant coined the
term island universe to describe a galaxy.
2. Astronomers now estimate that there are
100 billion galaxies in the observable
universe.
3. One of the earliest uses of the English
term Milky Way was in Geoffrey Chaucer’s
14th-century poem “The House of Fame.”
He likened the galaxy to a celestial roadway.
4. While we’re talking road trips: Due to the
expansion of the universe, all other galaxies
are receding from our own. Galaxies farther
from the Milky Way are speeding away
faster than those nearby.
5. Some of the galaxies receding from the
Milky Way are ellipsoidal, like footballs.
Galaxies can also be thin and flat with
tentacle-like arms — just like the Milky Way.
6. Galaxies come in irregular shapes, too,
including many dwarf galaxies. These
galaxies, the smallest in the universe,
contain only a few hundred or a few
thousand stars (compared with 100 billion
stars in the Milky Way).
7. You’ll often find dwarf galaxies clustered
around larger galaxies.
8. Dwarf galaxies frequently lose their stars
to their larger neighbors via gravity. The
stars stream across the sky as the dwarf
galaxies are ripped apart. Alas, you can’t see
it with the naked eye.
9. You also can’t see the enormous black
hole lurking in the center of the Milky Way,
though if you’ve ever looked at the
constellation Sagittarius, the archer, you’ve
looked in the right direction.
10. Most galaxies have a black hole at the
center, and astronomers have found the
mass is consistently about 1/1000th the
mass of the host galaxy.
11. Two of the closest galaxies to the Milky
Way — the Small Magellanic Cloud and the
Large Magellanic Cloud — may not have
black holes. Or, because both are low-mass
galaxies, their central black holes may be
too small to detect.
12. Every galaxy does have dust, though.
Produced by stars, the dust causes light to
look redder than it really is when observed
visually, which can make it difficult for
astronomers studying properties of stars.
13. That dust can really travel, too. Some
galaxies drive galactic winds, expelling dust
and gas at hundreds of kilometers per
second into the intergalactic medium, the
space between galaxies.
14. These winds are caused by starlight
exerting pressure on the dust and gas; the
fastest galactic winds are in distant galaxies
that are forming stars more rapidly than the
Milky Way.
15. The Milky Way rotates at about 250
kilometers per second (about 560,000
mph) and completes a full revolution about
every 200 million years.
16. One galactic revolution ago, dinosaurs
ruled the Earth.
17. Galaxies rotate faster than predicted
based on the gravity of their stars alone.
Astronomers infer that the extra
gravitational force is coming from dark
matter, which does not emit or reflect light.
18. Dark matter aside, galaxies are mostly
empty space. If the stars within galaxies
were shrunk to the size of oranges, they
would be separated by 4,800 kilometers
(3,000 miles).
19. If galaxies were shrunk to the size of
apples, neighboring galaxies would only be
a few meters apart. The relative proximity
of galaxies means that galaxies occasionally
merge.
20. In about 4 billion years, the Milky Way
will merge with the Andromeda galaxy. The
result of the merging process — which will
take at least a hundred million years — will
be an ellipsoidal galaxy nicknamed
“Milkomeda.”
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