00:00Is there a giant mega-sun in the center of our Milky Way?
00:05Scientists actually thought so for a while because it's the brightest place in the galaxy.
00:09It has millions of stars packed into a small area.
00:13This area is called the gigantic bulge.
00:17The gigantic bulge has millions more stars per light-year than any other part of the galaxy.
00:23It can be 10 million times denser than our part of the neighborhood.
00:27In some cases, stars in this region are only 5 light-days apart.
00:32That's like if there was another star in our solar system a bit further away than Pluto on a space scale.
00:38But why is this place so dense?
00:41When galaxies form, a lot of gas and dust come together under the force of gravity.
00:46This material gathers up and eventually forms stars.
00:50Gravity and angular momentum balance out, and it starts looking like a flat disk with a pretty bright, bulging, dense core.
00:58The stars live their comfy lives, and once they get to the finishing line, they collapse under their own huge weight.
01:05Then the black hole forms.
01:08Black holes love to eat everything around them, and the more they eat, the bigger they get.
01:13And what place is more perfect for a fine dinner than the galaxy's center, where all the space stuff is packed together?
01:21So it starts eating surrounding gas and dust, forming an accretion disk.
01:25As this material spirals into the black hole, it heats up and emits a lot of energy, which makes the center of the galaxy even brighter.
01:35That's why you can find both supermassive black holes and galactic bulges in the centers of all galaxies.
01:44The galactic bulge at the center of the Milky Way looks a bit like an ellipse.
01:49That's a classical bulge.
01:51Stars aren't like our Sun.
01:53They move randomly in all possible directions and planes.
01:57Plus, they all move at different speeds.
01:59So gravity is going crazy there, and this makes the bulge look more like a sphere, or an ellipse.
02:05Since they were the first ones to form, they have some of the most ancient stars in our galaxy.
02:11But there are also some places of star formations and lots of younger, massive stars that are less than a hundred million years old.
02:20As we move farther away from the center, things get a bit calmer.
02:24Stars start rotating uniformly and become stable.
02:28Right now, Earth is in one of the Milky Way's spiral arms called the Orion Arm, pretty far away from the galactic bulge.
02:36In our part of the neighborhood, stars are usually about four or five light years apart.
02:41This means that most of a galaxy is actually just black, empty space.
02:47Our black hole is called Sagittarius A star.
02:51It's a monster about four million times the mass of our Sun.
02:55It's also about 32 million miles in size, almost like the distance between Mercury and the Sun.
03:02But don't worry, it's not attracting the Milky Way inside it, and it's not gonna eat us.
03:07These black holes are actually super small compared to the entire galaxy, so they can only eat whatever's around.
03:14Right now, many stars orbit Sagittarius A star.
03:18And even though it emits a huge amount of energy, we can't see its light from Earth without a lot of scientific effort.
03:26But why don't we see the center itself?
03:29The galactic bulge is so bright that even though it's 26,000 light years away, we should see it shining brightly in our sky.
03:37Yet, we don't.
03:39Turns out it's all because of space dust.
03:42There's a lot of dust between us and the core, and it absorbs most of the visible light.
03:47We can only look at the galactic core using other types of light, like near-infrared, gamma ray, and so on.
03:55NASA has images of the core in different types of light, and it shows how scarily bright the center is without the dust blocking our view.
04:04But not all bright regions are blocked by gas and dust.
04:08For example, when we look at dense clusters like the Messier 13, the stars are so close together that they look just like a white spot.
04:17Most of our telescopes can't separate them from each other.
04:20The satellite galaxy M32, our neighbor near the Andromeda galaxy, has about 84 stars per light year.
04:28It's so dense that stars can't be resolved even by the Hubble Space Telescope.
04:34To get the idea, our solar system is two light years long.
04:38We'd see about 168 stars outside our window if we were there.
04:44Our closest star is Alpha Centauri.
04:47It's about four light years away from the Sun.
04:50If it was just a couple of light days away, it would shine much brighter than the full Moon.
04:56So if we somehow managed to survive in crazily dense star regions, the sky would be white all day long.
05:03But it's unlikely that we'd make it.
05:05As we get closer to the center of the Milky Way, the chances of finding life get super slim.
05:11The gravity of stars is going wild with chaotic movements, so there are barely any planets around.
05:17On those miracle planets, the radiation from cosmic rays is skyrocketing.
05:23Supernova blasts and star collisions nearby become an everyday occasion.
05:28And all the gas around makes it basically impossible to breathe or even see properly.
05:34At the same time, as we move further away, there are fewer stars around.
05:39Elements that are super useful for life like carbon, oxygen, and iron are produced by stars, so they also drop.
05:47Too much radiation is awful, but too low radiation means that there's not enough energy to support chemical reactions like photosynthesis.
05:55Which is why if there is extraterrestrial life, it would most likely be somewhere in the middle of different galaxies.
06:03But some galaxies get their brightness from a so-called active galactic nucleus.
06:09These are extremely energetic regions at the center of some galaxies.
06:14They shine much brighter than any stars imaginable, although it mostly shines only in certain parts of the electromagnetic spectrum.
06:22The brightness comes not from the stars, but from the accretion disk around their supermassive black holes.
06:30As the material slowly falls into a black hole, it gathers around it and creates this flat, spinning disk of gas, dust, and other stuff.
06:39Since the gravity and friction there are insane, this disk heats up and starts emitting enormous amounts of energy.
06:47Also, this disk spins incredibly fast, almost at the speed of light.
06:52Because of that, collisions there happen all the time, and they're unimaginably powerful.
06:58They release even bigger amounts of energy.
07:01Most galaxies don't have an AGN.
07:04Those that do, like the galaxy M87 in the Virgo constellation, are called active galaxies.
07:12There are also different types of AGNs.
07:16The Seyfert galaxies, radio galaxies, and finally, the winners of our space brightness competition, quasars.
07:24Imagine things so bright that they can outshine the entire galaxy they belong to.
07:29Quasars are a specific type of active galactic nucleus, the most extreme and luminous form.
07:36They belong to the supermassive black holes, the biggest ones in our universe.
07:41Quasars are like a combination of several things.
07:44First, they're the brightest accretion disks in our world because of their behemoth black holes eating everything around.
07:51But they have some cool features.
07:53For example, they have powerful jets of particles that shoot out from the poles of the black hole at nearly the speed of light.
08:01These jets add up to the brightness of the quasar, although they can only be seen in radio wavelengths.
08:08The energy they emit is so intense that they can be seen billions of light-years away.
08:13However, the nearest quasar to us is 600 million light-years away, so we can't see them with backyard telescopes.
08:21So, when we look at a galaxy through a telescope, we usually see only the brighter core, not the outer parts.
08:28Unfortunately, our eyes just aren't made to see things like the active galactic nucleus.
08:34So, these stars are the brightest things we can see.
08:38But what a beautiful sight it is!
08:40That's it for today.
08:41So, hey, if you pacified your curiosity, then give the video a like and share it with your friends.
08:46Or if you want more, just click on these videos and stay on the Bright Side!
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