M100: A Grand Design - June 6th, 1998.

Andromeda Through Gas and Dust

i don't know how to tell you how much i know,, but ,,, random question time: what's your favorite celestial body (and why)?

No need to stress over how to tell me how much you know! Since we're on my blog I'll just yap teehee. Choosing a favourite celestial object is *so* hard because there is so many to choose from, but space do be huge. And old. Before I get to my two specific favourites I'll do an overview of what the major objects in space are. I will not being going over constellations.

Planets and Moons

I'm going to introduce them jointly as most people are familiar with what they are as we live on a planet with a huge moon haha. According to NASA, Merriam-Webster dictionary defines a planet as "any of the large bodies that orbit the sun in the solar system." However this was further specified by the IAU in 2006- likely because the old definition was specific to our solar system and very broad. The IAU provide a planet must satisfy the three following requires.

It must orbit a star (originally the wording was the Sun but modern astronomers use star in place as again- the Sun is our specific star).

It must be big enough to form a spherical shape by gravity.

It must be big enough to clear objects from its orbital path. (NASA seems to add "of a similar size" to this as, for example, Jupiter has the Trojan Asteroids in it's orbit.)

Pluto was demoted because of rule three specifically as it's orbit goes right into the Kuiper belt; and the additional dfn provided by NASA makes that case stronger with its moon Charon.

Moons are just smaller objects that orbit planets- at least that's what I will say on them as this was longer than I meant it to be.

Side note: other objects in solar systems that are not moons or planets or...dwarf planets are asteroids, comets and meteoroids. These are basically left over material from the formation of our solar system. They don't get their own section despite the fact they deserve it as I'm an...interstellar->extra/inter-galactic space nerd, not a moon/planet/asteroid/etc space nerd.

Stars and Brown Dwarfs

These objects, but maybe more specifically the Sun's stellar physics, is what drew my autistic little ass into astronomy/physics to begin with, and while technically Brown Dwarfs aren't stars....against my better judgement and my friend who researches them, I'm lumping them together here. Stars are giant balls of hot gas that are able to fuse, "burn", Hydrogen into Helium and so on.

Brown dwarfs are not as hot as stars are but they aren't planets because they also do go through nuclear fusion- but instead of the normal hydrogen it fuses a limited supply of a heavier isotope of it; deuterium into helium and releases energy like a star (as per Caltech). These objects are dark in visible light but glow in infrared. After these objects burn up their deuterium, they tend to go into a cool contracting phase while glowing. These are commonly called failed stars but that's a bit of a misconception because these are not stars nor are they planets. It irritates the shit out of my friend who studies these objects when they are referred to as such.

All brown dwarfs and stars do fit into the Hertzsprung–Russell (HR) diagram, which tells us how young, how bright, and how hot each object is. That diagram cuts off at M stars traditionally but brown dwarfs continue the pattern. These objects are also classified by spectral type which goes by the same information as the HR diagram.

Spectral types from hottest/youngest to coolest/oldest: O B A F G K M || L T Y

Lty are brown dwarfs.

Nebula

These objects are star forming regions in space! Often referred to as stellar nurseries by people- there's kind of no better way to describe these hydrogen rich areas, I say rich because hydrogen makes up about 90% of the gas present in these objects. Two of the popular nebulae are the Orion Nebula and Eagle Nebula (contains the Pillars of Creation).

Planetary Nebula

So despite the name- these are actually not planet forming nebula, astronomers thought they were at first hence the name...but then we figured out what they actually were. The name just stuck (unfortunately imo). Planetary nebula are actually dead stars that are not massive (aka stars of spectral types A to M). These are formed when the star explodes, ejecting it's outter layers to the space around it and leaving the core (now a white dwarf) in the center.

As a G type Star, our very own Sun will have this fate when it explodes. In like five billion years- doubt humans will last that long

Black Holes and Neutron Stars

These beautiful objects are often considered to be the most extreme objects in our known universe, and both are the products of O and B type (supermassive/massive) stars exploding. These stars supernovas are much more violent because of how big they are so more extreme objects are left in it's wake. Black holes are characterized by the following.

Infinite density; yeah you read that right. It's hard to explain but basically at the center of the black hole there is a singularity- matter gets crushed down to an infinitly small point and the laws of physics as we know them break down. Aka since the gravity at this point is infinity we can't make meaningful calculations here.

Event Horizon; basically where the "black" part of the black hole begins. It is the surface of the black hole, but unlike the earth's or even the sun's, it is the boundary of which escape is impossible, even for light, which is why we see a sphere of black with light around it.

Black holes also have accretion disks, which are dust, rocks, and other matter that are swirling around/toward, typically, a black hole in the center. These are moving so fast that friction causing them to burn. However black holes are not the only objects with accretion disks; other massive bodies like protostars can have these too. Quasars are black holes with super fucking bright accretion disks that emit electromagnetic radiation, they are also referred to as AGN as they are the cores of galaxies. Most cores of a galaxy has a core that has gone through this short phase- including our own.

A common misconception: they are not like roombas sucking things into them, their extreme gravity is *pulling* things into it- it's like how meteors (that burn up in the sky) and meteorites (survived their burning) fall onto Earth.

Neutron stars are formed when the core is 1-3 solar massives (basically mass of the sun times 1-3) collapse, but the formed neutrons (from the protons and electrons smacking eachother in the formation) are strong enough to stop further collapse. Any star with more massive cores become black holes. These are characterized by the following;

Extreme, not infinite, density. Often these stars have masses greater than the sun condensed down into like. The size of a city. Super small, but super dense.

Critical case of the zoomies: but like...spinning zoomies. These spin super fast- and because they spin super fast they can emit beams of radiation making them into a Pulsar.

Strong ass magnetic field, like trillions of times stronger than the Earth's. This is because the magnetic field lines of the old star were compressed down- packing it much tighter which increases the magnetic flux (measure of field lines passing through an object- but let's simplify it more and call it field strength). If they are even *stronger*, like a thousand times stronger than the normal trillions, they are then Magnetars.

Star Clusters

There are three distinct types of these clusters ahem, groups of stars.

Globular Clusters: These are groups of really old stars that form and ar tightly packed together. These usually form in the outskirts of galaxies, known as the halos. These also have a more spherical shape than the other two. I think of these as like...the retirement homes of stars. A popular example of one is the Great Hercules Cluster (M13).

Open Clusters: These are groups of younger stars (with a few old ones) that typically form in the arms of spiral galaxies (galaxies like our own). They are not as tightly packed like globs, hence the name open. They're like the neighborhood full of kids and teens basically. A very popular example is actually the Seven Sisters! (Known also as Subaru or Pleiades). You can see that one with the naked eye.

Embedded Clusters: These are groups of very young and newly forming stars. Basically the hospital full of newborns and baby stars. These clusters have a lot of gas too. This cluster is the precursor to both open and globular clusters. A popular example is the Trapezium Cluster within the Orion Nebula.

Embedded Cluster vs Nebula: The difference really is just that the embedded cluster is actively forming stars from my understanding tbh- ECs are basically open clusters with a lot of gas still whereas nebula are just huge gassy areas containing these.

Galaxies

Arguably the biggest objects in space- these are collections of all of the other objects mentioned gravitationally bound together. Mind you the definition commonly given specifically mentions billions of stars and surrounding gas. Typically, a galaxy will also have a black hole at the center. There are also several different types of these.

1. Elliptical Galaxies: These galaxies are characterized by being more spherical/ellitpical with billions of old stars. Globular clusters on fucking crack basically. They have minimal star forming regions and new stars usually end up forming as a result of collisions.

2. Spiral Galaxies: Unlike the prior type, these are characterized by being a disk object with arms (those with more prominent arms are barred spirals) that extend from the dense center. They have a lot more gas than Ellipticals and this have more star forming areas. Thus meaning, they're younger. Both Andromeda and the Milky Way are spirals.

3. Irregular Galaxies: These are galaxies with no defined shape and can't be defined as elliptical or spiral. They're often usually small....but there are cases of huge ass ones.

There are galaxies referred to as Starburst galaxies, they can be any of the forementioned types. They're only characterized by the higher than average star formation rate.

My Favourite two Objects

I cannot choose between these two objects so I'll hand over both.

Sagittarius A*: This is the supermassive black hole at the center of our very own galaxy- so I have a little silly attachment to it. Even though there are closer black holes- it's even my profile picture! And and and! I've been to a telescope involved in the Event Horizon Telescope mission- which was simply to get a picture of Sag A*