# Astronomy glow-ssary!

# Introduction

Here is a list of the main terms commonly used in astronomy. We will try to keep all definitions simple and understandable while linking to trusted definitions on Wikipedia or other encyclopedia if needed.

# List

# Absolute magnitude

The absolute magnitude is the apparent magnitude of an object seen from 10 parsecs or 32.6 light years away, unobstructed by anything (particularly interstellar gas & dust clouds).

More: Wikipedia: Absolute magnitude.

# Amazing observation

For most amateur astronomers: capturing a faint fuzzy spot of what could pass for light if it wasn't that washed out.
For non-astronomers: wondering where they should watch and what to look for!

# Apparent magnitude

The apparent magnitude is a number that measure the brightness of an object as seen from Earth. The brighter the object the lower the number (and vice-versa). Negative magnitude is common (the Sun is -26.7 and the Moon is -12.6). In astronomy, it is used as a measure of visible light emission as well as UV and infrared emission (for astro-photography).

More: Wikipedia: Apparent magnitude.

# Apoapsis

This is the highest point of an orbit. You ascend to the apoapsis, once you pass it you fall to the periapsis. Apsis means orbit in Greek.
If you want to be really technical and look smart at dinners, it is technically the highest distance between the barycenter of 2 orbiting bodies (Earth and a spacecraft for example).

# Asterism

An asterism is what everybody calls a constellation: a popular and well known imaginary pattern that you can see in the apparent organization of the stars in the sky.
For example: the big dipper in Ursa Major. Big Dipper Credits & more: Wikipedia

There is many asterisms in many night lore like the Aboriginal astronomy, Islamic astronomy, Chinese constellation or Nakshatra.

# Asteroid

The definition for asteroid is pretty broad: anything that orbit the Sun and that is not a well defined planet and does not have the characteristics of a comet. Particularly: they do not evaporate when they get closer to the Sun.
This lead to differentiate the asteroids from the inner solar system from the objects of the outer solar system, or as it is called (another occasion to shine during dinners with big words): the trans-neptunian objects (everything that is further than Neptune).

Asteroids usually often refers to the objects in the asteroid belt and on Jupiter's orbit (the ones that haven't been cleared from existence by Jupiter gravitational pull at least...).

# Astronomical unit

It's a unit of distance (again). This used to be defined as the distance between the Sun and Earth. However, this distance varies with Earth highest and lowest points (respectively aphelion and perihelion).
In this time and age, vague and non-precise units are not favoured by anyone (cough cough aside from the 3 countries in the world still using the imperial system of weight and measures...), so an AU (abbreviation for Astronomical Unit) is now defined precisely as 149 597 870 700 metres.

More: Wikipedia: Astronomical Unit, Apoapsis, Periapsis.

# Barycenter

In geometry the barycenter of a figure is its center. In astronomy it's almost the same but also slightly different... It's the center of something at least!
It's actually the center of masses. That is to say that 2 (or more) bodies that revolve around each other (like Earth and the Sun), actually revolve around a point in between these 2 bodies. The position of the point depends on the mass of the bodies (hence the name: center of mass).
The actual definition is a bit more complex...

The barycenter of 2 bodies of same size is in between them:

The barycenter of 2 bodies with widely different mass is a lot closer to the most massive one:

To the point (like with Earth and the Sun), that it's inside the most massive body:

Fun fact: the barycenter can be external to all bodies:

All animation are from Wikipedia

# Binary star

Can you guess? It's of course, 2 stars orbiting each others. Well, actually they are orbiting their barycenter. The life of binary stars is not boring at all:

  1. they form,
  2. orbit each others,
  3. then one starts to vampirise the other,
  4. the vampirised one blow into a supernovae,
  5. the vampire grow into a red giant and then
  6. explode into a supernova!

All that is left is a neutron star, which is a collapsed core of star. That is the most massive type of star known to date (aside from black hole).

Some binary stars are actually visual double stars: they look like double stars but this is just luck alignment of 2 stars in the same field of view. They are not close to each other in space.

Of course, this is really simplified, go see the Wikipedia page for more accurate information and cool animations.

# Comet

A comet is a solar system object that is orbiting the Sun in eccentric elliptical orbit. If you are picturing an egg-like ball of ice with green and pink hairs you are spot on!
or are you? Eccentric, means that it is not really on the same plane than the rest of the planets and asteroids. This plane is called the ecliptic.
Elliptical, means that its orbit looks more like an egg than a nicely done circle.

When comets come closer to the Sun they evaporate and it generates tails. Comets have 2 tails. One is made of dust and trail behind the comet in a mostly curved path. The other is made of gas and is mostly going straight away from the Sun.

comet Credits and more: Wikipedia

# Constellation

That's a bit of a tricky one! The first meaning of constellation is coming from its latin root: a set of stars.
But in 1928, the International Astronomical Union officially adopted a list of 88 modern constellations. And the definition is slightly different from the initial "Look at the stars they look like a frying pan!".
Now the constellations are sky boundaries (that still contains the imaginary pattern we see in the stars) that split the night sky. Here is what a map of the constellations look like: Constellations Credit: Wikipedia

Of course, the Ursa Major constellation still contains the Ursa Major asterism (and the big dipper, by extension).

For the pretty imaginary patterns, see the definition of asterism.

# Dark matter

Easy: we don't know anything about it! Well, aside from the fact that:

  1. It exists (most probably).
  2. It account for about 85% of our universe mass.
  3. It does not interact with light and other electromagnetic radiations.
  4. It does not interact with "normal" matter except through gravity (it is therefor probably non-baryonic matter).
  5. It holds galaxies together.

That's about the extend of what we know. If you are interested to know more, have a look at Wikipedia's page.

# Galaxy

A galaxy is an accretion of stars, star systems, dust, interstellar gas and more importantly dark matter (allegedly). There is different types of galaxies: elliptical, spiral or irregular.
Galaxies comes in different size from the dwarves with merely e few hundred million stars to the hundreds of trillions of stars (like the giant IC 1011).

Our home galaxy is called the Milky Way and although it's one of the biggest of our Local Group, it is a relatively small one at the scale of the visible universe. The Milky Way is home to a super-massive black hole nicely named Sagittarius A*.

Galaxies are no exceptions to the relativistic gravitational attraction and boundaries. They are grouped into local groups and then super clusters (of galaxies not stars) and distributed in the universe along filaments.
The Milky Way for example if gravitationally bound to about 50 galaxies from our Local Group (including Andromeda with whom we will eventually merge in a couple billion years). Our own local group, is part of the Virgo Super Cluster. In turn this super cluster falls toward the Great Attractor a gravitational anomaly at the center of Laniakea, the local cluster of super-clusters. Feew!

NGC 4414

# Globular cluster

A globular cluster is a huge group of (a lot of) stars that are tightly bound by gravity. This bond is what gives them this sphere shape. They contains a lot more star than open clusters as we count them not in thousands but hundreds of thousands!
Globular clusters orbit the center of the galaxies (or galactic core). Scientific estimates a number of about 200 for the Milky Way, but our "nearest" galactic neighbor, Andromeda (2.5 millions light year away) hold about 500 of them, and Messier 87 (the galaxy from which we recently snapped a picture of a black hole) has around 13 000 globular clusters!
Globular cluster's stars are also very ancient, they are some of the first stars in the galaxies.

On a side note, they make excellent targets for amateur astronomers.

M80 Credits and more: Wikipedia: Globular cluster

# Light year

It's the distance crossed by the light in one year. Light travels roughly at 300 000 kilometers per second.
1 light year is equal to 9.46 x 10^12 km (or 5.88 x 10^12 mi).

# Magnitude

Magnitude is a measure of the brightness of an object in a specific spectrum. For example, visible light or infrared.
In astronomy we use 2 different definitions of magnitude: absolute magnitude and apparent magnitude. Why two? Because one is too easy to understand!

More: Wikipedia: Magnitude.

# Meteorite

A piece of space debris that falls on a planet or moon and survive the passage through the atmosphere.

When it encounter the atmosphere it transforms into a meteor (mostly because of frictions with the atmosphere). A meteor is commonly known as a shooting star.

# Nebula

Nice word to say: cloud. It's not your typical earth-like cloud, it's a cloud of dust, hydrogen, helium and some other gas (mostly ionized by stars or stars remnants).
Nebula are valuable targets for astronomers as they are mostly beautiful, like the pillars of creation in the Eagle Nebula for example (you need a space telescope to take that kind of photo): Eagle Nebula's pillars of creation Credits: Wikipedia

Fun fact about the pillars of creation: they most likely have already been destroyed by a supernova explosion, but the light from that cataclysmic event will take another 1000 years to arrive at Earth.

More: Wikipedia: Nebula

# Open cluster

An open cluster is a group (not more than a few thousands) of stars born from the same star nursery. They are loosely bound by gravity but this bound is relatively easily disrupted by encounters with other massive objects (like other clusters or gas clouds).
Open clusters are found in the disk of galaxies, further away from the center than their globular counterpart.

The most well know open cluster is the Pleiades cluster: Pleiades Credits: Wikipedia

More: Wikipedia: Open Cluster

# Parallax

Big word for a very simple concept: this is simply the difference in apparent position of an object depending on the viewer position. Put your finger up at arm's length, close your left eye and look at your finger and see what's exactly behind. Now, without moving close the right eye and open the left eye. Your finger remained in the same position but the background "moved". That's the parallax. In astronomy we tend to use the Sun as a reference point and either our position or an object position in space to define parallax (especially when defining units like a parsec).

# Parsec

A parsec is a distance unit. It is equal to about 3.26 light years. It is define as the distance from the Sun to an object that as a parallax angle of 1 arcsecond.

More: Wikipedia: Parsec.

# Periapsis

This is the lowest point of an orbit. You fall to the periapsis, once you pass it you ascend to the apoapsis. Apsis still means orbit in Greek.
If you want to be really technical and look even smarter at dinners, it is technically the lowest distance between the barycenters of 2 orbiting bodies (Earth and the Sun, but in this case we have special names Perigee or Perihelion).

# Quasar

Quasar comes from the contraction of quasi-stellar object. They are extremely bright galactic cores, most of the time because of a super-massive black hole coupled with a massive gaseous accretion disk. The gravitational strength generate so much frictions that a single quasar emits a lot of energy: up to a thousand times the total amount of energy emitted by our whole galaxy (The Milky Way)!

There is a lot more to say about quasars, I encourage you to read the Wikipedia page.

# Solar system

Just everything that is gravitationally bound to the Sun. That includes the planets (Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, Neptune and not Pluto), Dwarf planets (there you go Pluto, with Makemake, Haumea and the others), comets, asteroids belt, basically everything between the Sun and the outer belt of the Oort cloud.
The proposed boundary of the solar system is anywhere between 2 000 AU and 200 000 AU.

Keep in mind that the vast majority of the solar system is still uncharted territory.

# Star

I just love Wikipedia's definition: A star is an astronomical object consisting of a luminous spheroid of plasma held together by its own gravity.

All in all, it's a ball of gas (in a lot of case hydrogen) so gigantic that it crumbles under its own weight and light up nuclear fusion. The only thing preventing it to blow up in a cataclysmic event is its own gravitational pull.
It also burn tremendous amount of fuel each second, therefor radiating mass as light and heat. See the problem here?

In case it was not clear the Sun is a star...

# Star cluster

A group of stars usually bound by gravity. Some clusters just look like clusters from Earth but are not actually pack of stars in space, it's only perspective playing its game.
Within the real clusters you can find 2 families: the globular star clusters and the open star clusters.

# Star nursery

A Star nursery is a huge interstellar cloud where stars are born. This type of cloud is specific because it is dense and large enough to allow hydrogen to create molecules of hydrogen (H2).
The molecules of hydrogen locally aggregates and collapse into stars.

# Sun

Our good old Sun, the closest star from us.
The star the Earth is orbiting.

More: Wikipedia: Sun

# Surface Brightness

The surface brightness is equal to the magnitude divided by the apparent size of the object in the sky.
The bigger the number, the dimmer the object. The brighter the sky (because of light pollution), the more limiting this number is.

# Enhancing the list

This list is far from being exhaustive. Please feel free to suggest improvements either in the comments or even better: opening a PR on Github!
If you never heard of a pull request but still want to help, we have a tutorial for you!

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Last Updated: 8/23/2019, 3:03:00 PM