A mouse is about 30 grams.
Your average human is a lot bigger—about 70 kg.
An elephant is around 6000 kilograms.
Grams, kilograms…this is how we measure how massive something is here on Earth.
But what about out there...in SPACE?
Once you get to the size of something like a planet, it’s hard for us to comprehend
what the mass in kilograms even means.
That’s why astronomers often use their own special set of units for mass.
In our solar system, we have all these planets—8 or 9—or more!
depending on who you talk to.
Some of them are rocky, like Earth, and we know a lot about Earth, we’re comfortable
with its size—so we often express these other planets’ masses in terms of Earth
masses.
The symbol for Earth is a circle with a cross in the center.
Home sweet home.
The Earth’s mass is 5.97 x 1024 kg.
We can call this one Earth mass.
How many Earth Masses is the planet closest to the Sun, Mercury?
Mercury is 3.285 x 1023 kg, or 0.055 M⊕ “Earth masses.”
Let’s walk through that conversion: 3.285 x 1023 kg x (1M⊕ / 5.97 x 1024 kg) (kilograms
cancel) = 0.055 M⊕ Venus is 4.867 x 1024 kg or 0.815M⊕
Earth is 1 "Earth mass" by definition.
Mars is 0.107 "Earth Masses."
Does this help give you a feel for why gravity is so much less on Mars?
Way less mass means way less gravity.
Let’s continue our tour through the Solar System.
Now we’ve made our way out to Jupiter, a GAS GIANT.
Jupiter is more than 300 TIMES the mass of the Earth, and is by far the most massive
planet in the Solar System.
For this reason, we use it as another standard mass, especially for the big gassy planets
that are like Jupiter.
The symbol for a Jupiter mass is MJ {“m sub J”} or MJUP.
Jupiter’s mass is 1.898 x 1027 kg or 1 MJ “Jupiter mass”
Saturn is 0.3 MJ “Jupiter masses” Uranus: 0.046 MJ
Neptune: 0.054 MJ Astronomers also like to use this unit for
extrasolar planets and brown dwarfs. 51 Pegasi b is an exoplanet weighing in at
0.472 MJ The first brown dwarf announced was Gliese 229B, and it was estimated to be 20-50
MJ
Let’s not forget the very biggest thing in our Solar System—the SUN.
There’s nothing else close to this size in our own solar system, of course—but astronomers
use the mass of our Sun as a standard to draw comparisons with other stars.
We can also use solar masses to describe black holes, stellar clusters...or even entire galaxies.
For example, the mass of a typical spiral galaxy is on the order of 109 up to 1012 solar
masses.
One solar mass is about 2 x 1030 kg.
The Sun is represented by a circle with a dot in the center.
How do solar masses compare to our other astronomical standards for mass?
1 M☉ = about 333,000 M⊕, and about 1047 MJ.
Just to be clear—these masses that we’re throwing around here—we’re not MEASURING
them directly, like on a scale.
But we can CALCULATE these giant masses from other observable, measurable things, plugging
into Kepler’s Third Law.
By the way, did you know we had a video all about Kepler’s 3rd law?
We’ll link to it below.
So for instance, the mass of the Sun can be calculated based on the distance of the Earth
from the Sun, which is 1AU on average, and the orbital period of 1 year.
G is the gravitational constant.
Plugging in, we find MSun is about 2 x 1030 kg.
Would you guess that means our Sun is a VERY large star?
Remember we’re talking about MASS, not radius.
{include diagram of size ranges} In terms of mass, it IS more massive than most stars.
That’s not to say it’s THE most massive—astronomers have found stars with masses as large as 200
times that of our Sun.
One ENORMOUS star is RMC136a1, in the Tarantula Nebula, part of the Large Magellanic Cloud.
It’s estimated at about 215 M⊙.
We used to think when a star was that big, it just collapsed on itself.
Apparently not!
What about at the other end of star sizes?
Well, there’s a LITTLE problem if a star is too small.
Under 0.06 solar masses, nuclear fusion reactions can’t ignite.
{fizzle sound}
Other than these two extreme cases, why do we care how massive a star is, compared with
ours?
Well, knowing the mass of a star helps us predict its fate.
If the initial mass of the star is bigger than 8 times the Sun’s mass, the death of
the star results in a supernova (a very powerful explosion of the star).
Supernovas are not exactly rare events (although the last time you could see one with your
naked eye was Supernova 1987A, and before that, Kepler’s Supernova in 1604).
We see remnants of supernovae all over the place.
The Crab Nebula is an example of what is left behind after a supernova.
What a beautiful mess.
If, on the other hand, the star’s mass is lower than 8 times the mass of the Sun, then
when the star dies we get what we call a PLANETARY NEBULA—like this one, the Cat’s Eye Nebula,
found in the constellation Draco.
By the way, PLANETARY nebulae have nothing to do with the planets—remind me to tell
you more about them sometime.
By the way, PLANETARY nebulae have nothing to do with the planets—remind me to tell
you more about them sometime.
Socratica Friends, if you want to #LearnMore Astronomy, be sure to visit our website, socratica.com.
We have some special follow-up lessons you can ONLY find there.