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Course: American Museum of Natural History > Unit 2
Lesson 3: GalaxiesWhat Is a Galaxy?
Galaxies are titanic swarms of tens of millions to trillions of stars, orbiting around their common center of gravity. They also contain interstellar gas and dust. Galaxies show a range of shapes that astronomers group into three basic classes: spiral, elliptical, and irregular.
Spiral Galaxies have three visible parts: a thin disk composed of stars, gas, and dust; a central bulge of older stars; and a spherical halo of the oldest stars and massive star clusters.
Elliptical Galaxies have smooth, rounded shapes because the orbits of their stars are oriented in all directions. They contain little gas and dust, and no young stars. Like spiral galaxies, elliptical galaxies are surrounded by globular star clusters and dark matter.
Irregular Galaxies have a chaotic appearance, and are usually small. Their irregular shapes are probably due to recent disturbances — either bursts of internal star formation, or gravitational encounters with external galaxies.
The Milky Way
Our Milky Way is a barred spiral galaxy containing over 100 billion stars. The spiral disk of stars, gas, and dust is about 100,000 light-years across and 2,000 light-years thick — flatter than a pancake. The central bulge of stars is elongated in the shape of a bar.
The Sun orbits within the disk on the trailing edge of a minor spiral arm, about halfway between the galactic center and the visible edge. The Sun takes approximately 220 million years to circle the galaxy, and it has completed about 20 orbits since the solar system was born.
The Milky Way Galaxy resides in a neighborhood of a few dozen galaxies called the Local Group. They range in size from small dwarf galaxies to the large Andromeda Galaxy. Over time, these galaxies interact with one another, changing their motions and shapes. The long-term evolution of a galaxy is influenced by being part of a group. The Milky Way and the Andromeda Galaxy, our nearest spiral neighbor, are headed toward each other. In about five billion years, they may collide and merge. Eventually, our remote descendants could be living in a large elliptical galaxy.
Want to join the conversation?
where the spirals come from?(12 votes)
We believe that in the center of every galaxy rests a black hole and since we know that black holes do not allow anything to escape (ANYTHING, not even light), it sucks in all the matter around it and because there are star clusters surrounding the black hole, as the black hole sucks in the stars clusters gradually, it creates a vortex-like appearance, or a spiral, hence why they are appropriately named as spiral galaxies.
Hope this helped! :D(4 votes)
- What is an andromeda galaxy?(7 votes)
The Andromeda galaxy is a spiral galaxy which is the nearest major galaxy to the milky way.(9 votes)
What is the most kind of common galaxy(6 votes)- The most common kind of galaxy is an elliptical galaxy as most of the star clusters bunch up to cast an appearance that resembles a star. Another important fact to note is that most of the stars that make up the composition of elliptical galaxies are namely older, low-mass stars, hence why astronomers and scientists alike consider elliptical galaxies as the oldest of the three types of galaxies present: Irregular galaxies, Elliptical galaxies, and Spiral galaxies.
Hope this helped! :D(2 votes)
can a galaxy hold 1,000,000 to 100,000,000 mi stars(8 votes)
Yes, because galaxies are mostly made up of stars(1 vote)
why does it look like a old oreo(6 votes)
I think that is just the type of picture they took(2 votes)
why does an elliptical galaxy look like one giant star?(6 votes)- Elliptical galaxies are mostly made up of star clusters thereby making them look like a star.(1 vote)
- how is the black hole or a red giant created?(4 votes)
- A black hole or a red giant is created through a series of sequences that precede the result of a black hole/red giant.
The sequence proceeds as follows:
Interstellar medium, Star-forming Nebula, Protostar, Main Sequence, Red Giant/Red Supergiant, Planetary Nebula/Supernova, White Dwarf/Black Hole/Neutron Star, Black Dwarf.
The aforementioned sequences might be a little bit difficult to comprehend, so I will "decipher" them for you.
1 --> The process always begins with an Interstellar Medium, which is the space between stars and galaxies that is composed of many combinations of gases that can eventually transform into a star.
2 --> After the Interstellar Medium comes to the Star-forming Nebula, which, as the name suggests, forms stars, and is mostly comprised of dust and gas.
3 --> A Protostar is formed and is the next stage after the Star-forming Nebula due to violent activity that is very similar to that of a black hole and essentially, gathers the dust and cloud surrounding it to create an early form of a star.
4 --> A diagram is used to classify stars accordingly using multiple factors and some of those are temperature, luminosity (brightness), and color - sometimes mass. One of those "sections" is called Main Sequence and is a small, thin band of stars that live out most of their lives in this category as they have achieved equilibrium, unlike its predecessors.
The Main Sequence was the ultimate divider because it creates two paths that stars can take after their lives have been completed in the respective stage. If the star (after the stage of the Main Sequence) is a low-mass star, it will follow the shorter route, however, it will live longer than the high-mass stars (since high-mass stars utilize their energy unsparingly), who take a different, longer route than the low-mass stars.
5 ---> After the Main Sequence stage, if the star is a low-mass star, it will convert to a Red Giant (which our Sun will become in a few billion years after imploding upon itself and engulfing our planet).
However, if it is a high-mass star, the star shall convert into a Red Supergiant, the same as a Red Giant, except bigger.
6 ---> After the low-mass star has converted into a Red Giant, it will eventually start "shedding its skin," until all that is remaining is essentially the bare bones of a Red Giant (its core), surrounded by a beautiful cloud of dust and gas, the Planetary Nebula.
The next stage of a Red Supergiant, however, is a Supernova and unlike "shedding its skin" by slowly using up its energy, since the Red Supergiant is the evolution of a high-mass star, it expands to be so large that it resists the gravitational pull and causes an explosion, instead of an implosion, that we call a Supernova, which is also quite spectacular to look at.
7 ---> As the cloud of dust and gas (the stage we call Planetary Nebula) dissipates, the only remnant of the vanishing are the bare bones of the Planetary Nebula (the core) and although most of the light has "shed" away, the core does still cast light and we classify this stage as a White Dwarf, the very bottom section of the Hertzsprung-Russell Diagram.
The remnants of a Supernova are the result of two separate paths again and they are either a Black Hole or a Neutron Star. Do note that if after a Supernova, it converts into a Black Hole, the Black Hole would be the very last stage of a high-mass star and does not get to convert into a Neutron star and vice versa. This is considered to be the very last stage of a high-mass star before the cycle repeats.
8 ---> Although the high-mass stars' cycle has already ended, the low-mass stars' journey continues further to the very last stage after a White Dwarf, which would be the Black Dwarf and this is considered to be the very last stage of a low-mass star as it does not emit any energy and is practically just a black ball floating in space before its cycle also ends.
Hope this helped! :D(1 vote)
How will humans live when the galaxies collide?(3 votes)- its likely that humans will not be around in ~5 billion years when Andromeda collides with the milky way galaxy.(3 votes)
why dose it look like some sort of mold ☠️(4 votes)
How are the spirals on the galaxy created?(3 votes)
