What are the types of galaxies?

 

• The most widely used classification scheme for galaxies is based on one devised by Edwin P. Hubble and further refined by astronomer Gerard de Vaucouleurs. It uses the three main types, and then further breaks them down by specific characteristics (openness of spirals, size and extent of bars, size of galactic bulges). In this age of multi-wavelength observing, the sub-classifications also include markers for such characteristics as a galaxy’s star-formation rate and age spectrum of its stars.

1. Spiral Galaxies.

• Spiral galaxies are the most common type in the universe. Our milky way is a spiral, as is the rather close-by andromeda galaxy. Spirals are large rotating disks of stars and nebulae, surrounded by a shell of dark matter. The central bright region at the core of a galaxy is called the “galactic bulge”. Many spirals have a halo of stars and star clusters arrayed above and below the disk.

• Spirals that have large, bright bars of stars and material cutting across their central sections are called “barred spirals”. A large majority of galaxies have these bars, and astronomers study them to understand what function they play within the galaxy. In addition to bars, many spirals may also contain supermassive black hole in their cores. Subgroups of spirals are defined by the characteristics of their bulges, spiral arms, and how tightly wound those arms are.

2. Elliptical Galaxies.

• Elliptical galaxies are roughly egg-shaped (ellipsoidal or ovoid) found largely in galaxy clusters and smaller compact groups. Most ellipticals contain older, low-mass stars, and because they lack a great deal of star-making gas and dust clouds, there is little new star formation occurring in them. Ellipticals can have as few as a hundred million to perhaps a hundred trillion stars, and they can range in size from a few thousand light-years across to more than a few hundred thousand. Astronomers now suspect that every elliptical has a central supermassive black hole that is related to the mass of the galaxy itself. messier 87 is an example of an elliptical galaxy. There are some subgroups of ellipticals, including “dwarf ellipticals” with properties that put them somewhere between regular ellipticals and the tightly knit groups of stars called globular clusters.

3. Irregular Galaxies.

• Irregular galaxies are as their name suggests: irregular in shape. The best example of an irregular that can be seen from Earth is the small magellanic cloud. Irregulars usually do not have enough structure to characterise them as spirals or ellipticals. They may show some bar structure, they may have active regions of star formation, and some smaller ones are listed as “dwarf irregulars”, very similar to the very earliest galaxies that formed about 13.5 billion years ago. Irregulars are characterised by their structures (or lack of them).source-http://space-facts.com/galaxy-types/.

This is a galaxy without stars..

What you're seeing is the galaxy J0613+52.

A dark galaxy made up of hydrogen and dark matter— but no visible stars!

If it doesn't have any star, how was it discovered?

The discovery was a complete error. On January 2024, When a team at the Green Bank Observatory accidentally pointed their telescope to the wrong coordinates, instead of finding nothing, they detected a strong radio signal.

The signal came from an enormous cloud of cold hydrogen gas weighing a trillion solar masses.

How do we know?

Because neutral hydrogen emits a specific microwave radiation with wavelength of 21 cm, which is not affected by dust absorption.

Green Bank Observatory

Astronomers are still trying to find why this Galaxy doesn't have any stars. If it has, why aren't they visible?

One of the reasons might be that the gas is too thin and spread out to collapse and form stars.

It is also very isolated— there are no known galaxies within ~330 million light years of it.

That's staggering, even on cosmic scales! For comparison, distance between Milky Way and Andromeda is ‘mere’ 2.5 million light years.

When galaxies interact and collide, it churns their gas and triggers star formation. J0163+52 never got a chance.

What's interesting is that this galaxy might be a “primordial galaxy”— an incredibly rare phenomenon when an object made up of gas remains largely unchanged since the early universe.

We shot in dark, with a couple of wrong coordinates, and ended up finding a galaxy which we won't have found if we relied on light. Just imagine how many hidden galaxies there might be spread across the universe!

Space isn't just “out there”. It is full of mysterious places waiting to be explored… who knows what we’ll find?

If Earth was shrunk down to the size of a grape, how big would the Galaxy be?

 Let's round the diameter of a grape up to about one inch (2.5 cm), just for simplicity's sake.

The diameter of the Earth is about 7917.5 miles (12,742 kilometers), which translates to 501,652,800 one-inch grapes. Give or take. That is to say that the Earth is currently 501,652,800 times bigger than a grape, so in order to make them the same size, we would need to scale down the Earth by 501,652,800 times.

We now assume that the same proportional shrinkage takes place in all of space, or at least our galaxy.

Doing this makes the solar system at least navigable. The Sun, which is now about the size of a Smart Car, is a little less than 1,000 feet (about 300 meters) away from our little grape. That’s a short little jog, or about the height of the Eiffel Tower. Be sure to stop to smell Venus on the way — it smells like rotten eggs!

Pluto is still over seven miles (over eleven km) away from the Sun, so we’ll need a vehicle if we aren’t in the mood to hike. Just to arrive at an icy pebble about 1/5 of an inch (1/2 of a centimeter) in diameter. You may (or may not) have noticed Jupiter looking like an overinflated basketball along the way. More likely you noticed Saturn and its spectacular rings, now just under two feet across.

But we’re not thinking big enough. Granted, the Milky Way is a relatively small galaxy, with a distance of "only" about 100,000 light-years across. Scaling that down by the same ratio, the galaxy would now be 0.0002 (1/5,000) light-years across.

To turn that into miles, we must consider that one light-year in reality is almost 5.9 trillion miles (9.5 trillion km). In our scale model, that same light-year is now a little more than 11,718 miles (18,859 km) -- a little greater than the distance from the United Kingdom to New Zealand.

So already we’ve outgrown our car. We’ll need a commercial plane and most of a day to travel a single light year. But if we want to get anywhere, the nearest star will require that we make that trip four times and then some, so we’ll need a high-speed jet with enough fuel to travel completely around the Earth (the real Earth) twice without refueling. (One commenter suggested a rocket, which is probably a better time-saver).

We’ve now made it to the Alpha Centauri system. Congratulations. That flight was probably miserable.

But we’re not even getting started. We’re trying to traverse the galaxy, after all, and we’ve only jumped from one star system to its nearest neighbor.

This scaled-down galaxy is now 1,171,826,372 miles (1,885,871,741 km) across. Essentially we’ve shrunk the Milky Way to fit inside our Solar System; the edges of the galaxy would be found somewhere between the orbits of Saturn and Uranus — still twenty-five thousand times further than humans have traveled thus far. If you want to get a feel for how long that trip would take, I recommend this map (which shows that even light itself would take more than an hour and a half to cover the distance):

If the Moon Were Only 1 Pixel

Even at over 500 million times smaller than actual size, our galaxy is still unfathomably massive.

Moral of the story: I hope you like your little grape. It’s almost definitely the only grape you’ll ever get. Do take care of it.

Note: the above calculations are completely linear. I only used diameter. If accounting for volume, the numbers would be different. For example, I had said that Earth is about 502 million times larger than a grape, but if measured by volume, Earth becomes more than one septillion times larger than the grape… (that’s a 1 with 24 zeroes). To make that useful we’d have to compare it to the volume of the galaxy, but I don’t know how to measure that (and I don’t think anyone else does either). Because I wanted to talk about travel times anyway I felt a linear measure was sufficient.

The Glowing Heart of Galaxy NGC 1097

 Take a moment to admire this stunning infrared image captured by the Very Large Telescope, which has an impressive 8.2-meter mirror. At the heart of the photo lies a bright inner ring of stars encircling the core of the galaxy NGC 1097. This fascinating galaxy sits about 45 million light-years away from us in the constellation Fornax.

The darker regions visible in the image show clouds of gas and cosmic dust spiraling toward the supermassive black hole at the galaxy’s center, slowly feeding its immense power.

Are there any areas in the Milky Way that are unlikely to have life?

 

As noted in the answer by Tom Nathe, the galactic core is too tightly packed with stars to have conditions that would be conducive to life. The proximity of stars to each other means that the orbits any planets around these stars would be disturbed by gravitational interactions with other stars, and thus would likely not have stable orbits.

Because the core of the galaxy is not usually thought to be a good place for habitable worlds, some astrobiologists identify a “galactic habitable zone” (GHZ), which is an habitable zone for the entire galaxy, analogous to the habitable zone around a given star, which latter is then referred to as a “circumstellar habitable zone” (CHZ). Any planetary system outside the GHZ would be considered an unlikely place to find life.

Globular clusters—small mini-galaxies of thousands to millions of stars that orbit larger galaxies—are similarly closely packed with stars and so pose problems for planetary habitability. A recent paper examined this: “Habitability in the Omega Centauri Cluster” by Stephen R. Kane and Sarah J. Deveny.

Another problem with globular clusters—a problem from the perspective of being clement to life—is that the processes of galactic ecology that work in the main body of the galaxy do not work in globular clusters. Galactic ecology is what we call the recycling of material from stars that explode in a supernova, scattering their remnants, which are then later incorporated into later generations of stars, which as a consequence have a higher level of heavier elements (both due to nucleosynthesis while the former star was fusing elements in its core, as well as new elements created by the supernova event itself). Planetary systems that incorporate more heavy elements (i.e., are higher in metallicity) are likely to be more minerologically and hence more geologically complex, and it is likely that geological complexity plays a role in the emergence of life.

An important caveat to the above: it should be observed that conventional conceptions of habitability have been questioned recently as we have learned that moons in our solar system (and probably also planets elsewhere in our galaxy) have large subsurface oceans under kilometers of ice exposed to the cold and vacuum of space. It is possible that life could arise in subsurface oceans, in which case the requirement of a planet being in a CHZ where liquid water could be found on its surface may be too narrow a criterion for searching for life and for the definition of a habitable zone.

If we reconfigure the idea of a habitable zone to allow for life in subsurface oceans, and perhaps also to allow for kinds of life radically different from life on Earth, the GHZ may be much larger than in the illustration above, and it may, in fact, include all regions of our galaxy.

Biggest Stars of our Milky Way Galaxy

 Imagine the biggest star we know, called UY Scuti. This star is so huge that if we placed it at the center of our solar system instead of the Sun, its outer layer, called the photosphere, would stretch all the way out to where Jupiter orbits. In other words, it would be even bigger than the Sun and would engulf space up to Jupiter's orbit, which is really far from the center of our solar system.

But that is not all, there are various factors taken into consideration while determining the size of a star, and some of them are.

1. Mass: The amount of matter a star contains is a crucial factor in determining its size. Stars with more mass tend to be larger.

2. Age: As stars age, they can change in size. Some stars expand as they evolve, becoming larger as they reach different stages of their life cycles.

3. Composition: The elements and gases that make up a star also play a role. Stars with different compositions can have varying sizes.

4. Temperature: The temperature of a star's core can affect its size. Higher core temperatures can lead to more intense nuclear reactions, which can either cause a star to expand or contract.

5. Gravity: The gravitational force acting on a star can influence its size. Stars with stronger gravity tend to be more compact, while weaker gravity can allow a star to expand.

6. Internal Pressure: The balance between the gravitational force pulling a star's matter inward and the pressure from nuclear reactions pushing outward determines its size. When these forces are in equilibrium, a star has a stable size.

These factors interact in complex ways, leading to a wide variety of star sizes and shapes in the universe.

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.