What Galaxy do we live in?

 We are living in a galaxy that we simply call “The Milky Way”. It is a middle sized spiral galaxy, or more precisely a barred spiral galaxy, with spiral arms and a kind of straight structure in the middle.

I cannot show you a picture of the Milky Way from the outside, since we are all living on the inside of it and we have not yet been able to send a space probe to the outside. But there are other galaxies that are of the same type, and they all look similar to the picture below (which actually is a computer rendering of our Milky Way).

The Milky Way have two distinct spiral arms, and a few additional, less bright and less clear arms or “spurs”. Our Sun lies near a small, partial arm called the Orion Spur, located between the Sagittarius and Perseus arm, in the “suburbs” so to speak. If the Milky Way was New York and the galactic center was Lower Manhattan, we would be living in Poughkeepsie.

The Milky Way is part of a small cluster of galaxies, called (perhaps not so imaginatively) “The Local Group”. The biggest galaxy in the neighborhood is the Andromeda Galaxy, which is in fact slightly bigger than our own. The Triangulum Galaxy is much smaller, and than there are a bunch of smaller, irregular dwarf galaxies.

As it happens, the Milky Way and the Andromeda Galaxy are on a collision course with each other, and are expected to collide in about 4.5 billion years.

This sounds more dramatic as it is - in fact, the galaxies contain so much empty space, that two galaxies can easily pass through each other without any stars actually colliding. Earth, or whatever is left if it, will probably not smash into something nasty. A stellar collision is less likely than two flies buzzing around in St. Peter’s Basilica colliding with each other in midair.

But the two galaxies will still eventually capture each other with their gravity and go through a series of oscillations to finally merge into one massive, distorted galaxy, like the two galaxies being in such a merge in the Hubble photo below.

Another cool fact is that in the heart of the Milky Way is a supermassive black hole, called Sagittarius A* (yes, it is actually pronounced a-star).

When this black hole was still young, it was still surrounded by gas clouds and stars. Having a black hole in this area was a bit like a placing a hungry tiger amidst a flock of sheep. It went into a feeding frenzy, devouring everything in its vicinity, growing for each swallowed star, belching out leftover matter and radiation that could not fit into its gaping throat at once. During this time, our galaxy was probably what astronomers call a “quasar”: the most luminous objects that ever existed, shining brightly as a beacon through the entire Universe.

Sgr A* however is now sleeping: it is dormant, dark and docile, having devoured everything in its vicinity a long time ago.

Frightening as a supermassive black hole in the center of our own galaxy might be, the Milky Way does not revolve around it. In spite of the black hole’s incredible mass, it dwarfs to insignificance when compared with the entire galaxy. Its gravitational effect on the galaxy is comparable with one of your eyelashes affecting your body with its mass.

So what holds the galaxy together? Why does it not fall apart? Given their incredibly fast rotation, galaxies should in fact fly apart into a thousand pieces, like a marshmallow attached to a lathe.

This is actually a very tricky question, and one that we still do not have a definite answer to.

We however believe that all galaxies are held together by something called “dark matter”. This is not just ordinary stuff that happens to be “black”, or “stuff that is in the shade”, but something fundamentally different. Dark matter does not interact with any electromagnetic force. You cannot shine a light onto it. It does not cast a shadow. It is in fact fully transparent. It is all around you as you read this, but you cannot observe it in any way. The only thing it has is gravity.

We today believe that dark matter is the most usual “stuff” in the entire Universe. And it does not get diluted as the Universe expands since it is most likely concentrated inside the galaxies, which do not get affected by cosmic expansion on a local level.

What are the types of galaxies?

 There are three main types of galaxies:

1.spiral galaxies:which have flat, rotating disks with spiral arms

2.elliptical galaxies:which are oval-shaped and contain mostly older stars

3.irregular galaxies:which lack a defined shape and often appear chaotic.

A Groundbreaking Discovery Within The Milky Way

 

Astronomers have made a groundbreaking discovery within the Milky Way—a colossal black hole, weighing 33 times more than the Sun, now holds the title of the largest known stellar black hole in our galaxy. Named Gaia BH3, this cosmic behemoth was detected using data from the European Space Agency’s Gaia mission, along with follow-up observations from ground-based telescopes. What makes this finding even more astonishing is its proximity to Earth—just 2,000 light-years away—placing it among the closest known black holes in cosmic terms.

Formed from the catastrophic collapse of a massive star, this black hole’s immense gravitational pull warps spacetime so intensely that not even light can escape. Its existence challenges existing astrophysical models, as scientists previously believed stellar black holes in the Milky Way rarely exceeded 20 solar masses. The discovery of Gaia BH3 raises new questions about how such massive black holes form and whether they play a role in the evolution of even larger, supermassive black holes at the centers of galaxies.

This hidden giant, lurking silently in the constellation Aquila, serves as a powerful reminder that the universe still holds many secrets. Black holes like this one continue to reshape their surroundings, influencing star formation and galactic dynamics in ways we are only beginning to understand. As astronomers study Gaia BH3 further, it may unlock crucial clues about the life cycles of stars, the formation of black holes, and the unseen forces that govern our galaxy.

What galaxy do you think is the most interesting?

 These are galaxies that I think are the most interesting. I got them as background photos on my phone and computers. It’s best to view them by clicking on each image so that it’s zoomed. You can then see intricate and intriguing details that will make you fall in love with The Universe. The best parts are some shadows from dust that are visible best in the galaxies that are shown from the side like the last one.

The Black Eye Galaxy, 17 million light-years away, the Coma Berenices constellation:

The galaxy NGC 4526 with a supernova explosion at the bottom left corner, 55 million light-years away.

The galaxy NGC 4414, 62 million light-years away.

The galaxy NGC 4846, 65 million light-years away.

The galaxy NGC 7049, 150 000 light-years across, 100 million light-years from Earth.

The Lindsay-Shapley Ring, 300 million light-years away, 150 thousand across, constellation Volans:

Interacting with each other the Mice Galaxies, 290 million light-years away, constellation Berenices:

The Needle Galaxy, 30–50 million light-years away, Coma Berenices constellation:

The Pinwheel Galaxy, 21 million light-years away, Ursa Mayor constellation:

The Southern Pinwheel Galaxy, 15 million light-years away, constellation Hydra:

The Sculptor Galaxy, 90 000 light-years across, 11 million light-years away, the constellation Sculptor:

The Sunflower Galaxy, has about 400 billion stars, 30 million light-years away, the constellation Venatici:

The Tadpole Galaxy, 420 million light-years away, the constellation Draco:

The Backward Galaxy, 111 million light-years from us, the constellation Centaurus:

The Mayal Object, 500 million light-years away, the constellation Ursa Mayor:

The Eye of Sauron assembly of galaxies in the constellation Canes Venatici:

The Circinus Galaxy, 13 million light-years away in the constellation Circinus:

The Butterfly Galaxies, 60 million light-years away, in the constellation Virgo:

The Bode’s Galaxy, 12 million light-years away, 90 000 light-years across, in the constellation Ursa Mayor:

The Cartwheel Galaxy, 500 million light-years away, 150 thousand light-years across, the Sculptor constellation:

The Whirlpool Galaxy, 23 million light-years away, the constellation Canes Venatici:

The Antennae Galaxy, 45 million light-years from us, the Corvus constellation:

The Fireworks Galaxy, 25 million light-years away, the constellation Cygnus and Cepheus:

The Galaxy Arp 273 interacting galaxies, 300 million light-years away:

The Galaxy Arp 142 interacting galaxies, 326 million light-years away:

The Galaxy Messier 94, 16 million light-years away.

A really good image of the Andromeda Galaxy. 2.5 million light years away:

The Galaxy M87 with a quasar jet, 53 million light-years away.

My all-time favourite is the Sombrero Galaxy, 31 million light-years away, on the border of the Virgo and Corvus constellation:

All Images from Hubble Space Telescope, Wikipedia.

Have some black holes already swallowed entire galaxies?

 Stars in a galaxy? Say, 100 billion, typical. But some are very small, down to 100 million.

Our galaxy supermassive blackhole, Sag A*, is only 4 million solar masses. The Milky Way has over 100 billion stars with a lot of them, many more times massive than our Sun. in fact the Milky Way is estimated at over 1.5 trillion solar masses. Did Sag A* eat a galaxy? No.

So, how about the most massive black hole, TON 618? That is 66 billion solar masses. That does put it in range of the smaller galactic masses. However, there is the problem of velocity. Things are not just floating around in space. Actually, everything has massive velocity. in one sort of orbit or another. The sort of orbit for galaxies, is a complex spiral, around all the galaxies around it. So, just imagine what this means. We can measure the proper velocity of the Milky Way, against the Cosmic Microwave Background. We are moving at 1.3 million miles per hour across the CMB. At these velocities black holes cannot eat everything, but they will gulp the part they are exposed to.. This means, the answer is no, black holes don’t swallow galaxies. Then how do they get so big?

They are part of the formation of some types of galaxies, as they undergo collisions with each other. They represent the core stars and dust collapsing, and merging, as another galaxy passes through it. Then, those galaxies collide. That can result in much more consumption of stars, and even the merger of the core black holes, over millions of years, of course.

What is the most mysterious object in our galaxy?

 It’s hard to say what’s the absolutely most mysterious, but the first thing that comes to mind is Eta Carinae.

The star (or stars - it appears to be a multi-star system) is among the brightest in the galaxy, with the primary component more than 5 million times brighter than the sun. At 7,500 light years away, it’s one of the most distant stars we can see without a telescope, and was the second brightest star in the night sky for a few days in March 1843.

Eta Carinae’s main component is highly active, and spewed a huge eruption of material in the mid-1800s. It was this eruption that made it so bright. It then faded so much that for a while a telescope was needed to see it. Now it’s visible again with the naked eye.

Scientists think it’s likely to explode in a supernova sometime in the near future (astronomically). That means anytime between now and say, 3 million years from now. If it does, it will be one of the brightest objects in our sky for a short period. Probably five times brighter than the planet Venus.

Despite its great distance, it might be one of the most dangerous objects in the sky, because it’s likely to be massive enough to explode as a “hypernova,” Some scientists predict it could strip the Earth of its ozone layer, something that wouldn’t normally happen unless a supernova were within 50 light years of Earth.

The Eta Carinae system is seen below. The material expelled from it during its eruption in the mid-1800s is quite evident. It’s this material that caused the star to fade in brightness from our vantage point for a while, as the material hid the star.

If the sun suddenly became as luminous as Eta Carinea (this can’t happen), the Earth’s entire surface would melt, mountains and all.

How is our Galaxy coincidentally exactly 100,000 light years across?

 It’s not exactly that distance across.

Wikipedia says that it’s size is 87,400 ± 3,590 light years.

That’s an accuracy of ± one part in 25.

So in other words - they’re saying that we know that it’s at least 83,810 light years and no more than 87,400 light years.

However - we can’t send spacecraft far enough away to take a nice photo of it like this for us to measure…

…and even if we could - you can see that the galaxy has VERY fuzzy edges and an extremely irregular shape.

Furthermore - we can hardly see anything on the other side of the central bulge of the galaxy because the dense stars, gasses and interstellar dust is blocking our view.

Quite honestly - I’m shocked that any “official” measurement could possibly be as accurate as stated in the Wikipedia article…after all - if there is just one star that’s a part of the galaxy that’s further out than that - then the answer is wrong.

GIven the massive scope for error - I suspect that whatever source you saw for the 100,000 light year number was just rounding it to a single digit of accuracy - which is an entirely justifiable thing to do under the circumstances.

It’s not some very accurate (and therefore amazingly coincidental) number - it’s a very, very ill-defined number that’s been (justifiably) rounded off to a realistic amount of precision.

The number that Wikipedia uses comes from a book written in 1998 - so it’s possible that there are newer numbers - is also has note that this is measured using the “isophotal diameter”…which it then explains in painful detail:

The isophotal diameter is introduced as a conventional way of measuring a galaxy's size based on its apparent surface brightness. Isophotes are curves in a diagram - such as a picture of a galaxy - that adjoins points of equal brightnesses, and are useful in defining the extent of the galaxy. The apparent brightness flux of a galaxy is measured in units of magnitudes per square arcsecond (mag/arcsec2);

Yeah - so to try to unpack this…the “isophotal diameter” doesn’t mean “if you measured from the furthest star to the east to the furthest star on the west - this is the number you’d get”.

CONCLUSION:

You can’t accurately measure the diameter of a weird shaped fuzzy blob with spiral arms - especially not if you have to do it using photos taken from a single point INSIDE that fuzzy blob!

Therefore, the number “100,000 lightyears” is rounded off to one significant digit - which is a very reasonable thing to do in the face of so much uncertainty.

Does every Galaxy have a black hole? Is that what keeps galaxies a Galaxy?

 There are many Galaxies with no detected black holes (BH). However, as the saying goes, absence of evidence is not evidence of absence. The fact is that it’s very hard to detect any single object at galactic distances unless it’s extremely luminous. In fact, we can only detect those BH’s that produce powerful astronomical jets. These jets are made up of a collimated beam of high-speed particles (see illustration below). To produce these jets a BH must be actively “swallowing up” large amounts of matter. But in older galaxies the BHs already swallowed everything in its vicinity (i.e. cleared its neighborhood). So there is no jet and they become nearly undetectable.

However, this is not a permanent state. If we wait long enough (perhaps thousands or millions of years), an unlucky star or nebula will eventually stray too close to the BH, fall into it, and produce a new jet.

Q. Is that what keeps galaxies a Galaxy?

The OP’s second question is much easier to answer: A typical galaxy has many millions of BHs of various sizes, among them is often a supermassive BH at the galactic center. As far as we know, we can remove all of them, and almost nothing will change in the structure or dynamics of the galaxy. They are just there, adding some small fraction of gravitational pull, but not much else. They are not needed to keep the galaxy a “Galaxy”.

What is the largest galaxy in the known universe?

 THE ALCYONEUS GALAXY

Astronomers just found the largest galaxy ever discovered, and they have no idea how it got so big.

At 16.3 million light-years wide, the Alcyoneus galaxy has a diameter 160 times wider than the Milky Way and four times that of the previous title holder, IC 1101, which spans 3.9 million light-years, researchers reported in a new study. Named after one of the mythical giants who fought Hercules and whose name means "mighty ass" in Greek, Alcyoneus is roughly 3 billion light-years from Earth.

The galactic monster is a particularly large example of a radio galaxy, or a galaxy with a supermassive black hole at its center that gobbles up enormous amounts of matter before spitting it out in the form of two massive jets of plasma moving at near-light speed. The plasma beams slow down after traveling millions of light-years, spreading out into plumes that emit light in the form of radio waves. The lobes of Alcyoneus are the largest ever discovered.

Galaxies with massive, plasma-filled radio lobes aren't uncommon (the Milky Way has two small plumes), but scientists are baffled as to how Alcyoneus, a relatively ordinary galaxy at its core, was able to grow such monstrously huge plumes.

Apart from its massive plumes, Alcyoneus is a typical elliptical galaxy, with a total mass roughly 240 billion times the mass of the sun (half that of the Milky Way) and a central supermassive black hole 400 million times the mass of the sun (100 times less massive than the largest black hole).

In fact, Alcyoneus' center is small in comparison to the centers of most radio galaxies.