Galaxies

 

Previously we saw about the Milky way galaxy. It's a type of spiral galaxy with huge arms and the solar system is located in one of the smaller arm called the "Orion arm". Galaxies are divided into Elliptical, Spiral, Irregular and peculiar based on their shape. But galaxies can also be classified on the basis of their behaviour,mass and location. Now let's dive deep into the Cosmos!

In the 1960s a peculiar object was found which was named as 3C273. While observing through an optical telescope it was thought to be a blue star, but while observing through a radio telescope it looked very bright like a luminous powerhouse. By spectroscopy it was found that it wasn't a star but an entire galaxy 2 billion light years away. It was one of the most luminous objects observed in the sky at that time emitting 4 trillion times the energy of the Sun. It was called a Quasar. Later many other similar objects were found in the sky that emitted an immense amount of energy. With the help of X- ray observations, these objects were found to emit X - ray radiations. Some objects were also found emitting Gamma rays, the very highest energy emitting light. Astronomers named these galaxies as "Active galaxies". They were categorised on the type of light they emitted or their spectra. It turns out that to create such energy we needed an object with a lot of gravity. These were none other than "Black holes". 

In the 1980s astronomers thought that these galaxies enjoying such high energy had Super massive black holes at their centers. These Super massive black holes grew as their host galaxies. When  materials coalesce to form galaxies, some of them fall to the center and feeds the black hole. If black hole sucks everything, even light itself, one would think how the same doesn't happen to us?  The answer to that question lies in the fact that when the objects are still outside the event horizon of the black hole things can still get out. So there wouldn't be much of a difference if the Sun is replaced by a black hole, except we would freeze to death. When some matter, dust or a whole star falls into the black hole these matter disintegrates due to the immense gravity and forms the accretion disk. This matter falls into the black hole like water down the bathtub drain. Stuff closer to the black holes has higher orbital speed than stuff farther from the black hole. The orbital speed around the black hole is near the speed of light. When the matter in the accretion disk rub against each other at such speeds, they produce immense amounts of heat energy. This heats up upto a million degrees causing light to blast out across the electromagnetic spectrum. This powers an active galaxy.

Active galaxies are so bright that they are seen clearly across the universe. Some of them also emit jets of twin beams of matter and energy, due to the magnetic field created by the incredible rotation of the accretion disk. This twin beams of matter and energy is emitted from the two poles of the accretion disk. Active galaxies look pretty different from each other. Scientists suspect this is due to our viewing angle of the accretion disk.

When we see them edge on, the matter and dust around the accretion disk blocks our view of the highest energy source of an active galaxy. If the twin beams are turned a bit to our line of sight, we see more high energy light from it. If the twin beams are aimed right at us we can see the ridiculously energetic X ray and Gamma ray light from it.

The Milky way galaxy has a super massive black hole of mass 4 million times as that of our Sun and it's not actively feeding. So our galaxy is not an active galaxy. But this black hole may change into an monster through galactic collisions. When two galaxies collide the gases around them collapses to the center and is gobbled up. Can the same happen to our galaxy? Yes it can. In fact, it will.

But not for a few billion years. Our Milky Way galaxy isn't alone. It's a part of a few knot of galaxies called the "Local group".

It consists of a few dozen galaxies, but they are all quite dim. The only major galaxies in the local group is The Milky Way galaxy and the Andromeda galaxy. Astronomers suspect that the local group has lots more galaxy, but they were consumed by the major galaxies which grew in size. The Andromeda galaxy is bigger than ours, with a supper massive black hole at it's center too, with a mass of 40 million solar masses*, 10 times that of ours.

The Andromeda galaxy is about 2.5 million light years away from us, close enough that it can be seen in the night sky. Spectra taken from Andromeda reveals an interesting fact,

IT'S HEADED RIGHT FOR US!!

Let's discuss more about the Milkomeda in the next blog.

Solar masses - It's a term used to refer to masses in space. One solar mass = mass of the Sun.