On April 20th (4/20) at midnight Earth’s horizon (in northern latitudes) lines up perfectly with the plane of the entire Milky Way galaxy. The galaxy “wraps around” our view of the sky and we can see into and through the plane of the galaxy.
The Milky Way galaxy lines up perfectly with your local horizon view on April 20th (4/20) at midnight each year.
At this moment when you look out around you at the horizon, you are looking into the plane of the Milky Way galaxy.
How to See the Milky Way Galaxy Plane
Look southward (and a little bit east) and you are looking toward the center of the galaxy. Look northward (and a little bit west) and you are looking toward the outer edges of the galaxy.
Why does the galaxy line up with the horizon?
Each year on 4/20 the galaxy lines up with the horizon – an “event” I call the Galaxy Horizon. In fact, this “event” occurs every day! It’s just an interesting coincidence that the alignment happens on 4/20 at midnight. Each day the galaxy lines up again with the horizon but it happens 4 minutes earlier.
Have you ever looked up and wondered how many stars are in the sky?
How many stars are in the sky right now?
It’s a simple question with a surprising answer!
This question has layers of answers – the most common answer is “it depends” – it depends on where you are, what part of the year it is, what time it is, and most of all… how dark are your skies and how powerful is your telescope?
Let’s start with a quick (approximate) list of how many stars you might be able to see at once at night under “normal” conditions with no telescope. (The details on these calculations are near the end of this article.)
How many stars can I see at night:
The most stars anyone can see from the Earth (no telescope): 3700
The number of stars can you see at a dark sky site: 2000
How many stars can I see in my suburban yard: 200
How many stars can I see in a large city: 20
Another trick answer to “how many stars are in the sky?” is: “all of them!” All of them are in the sky, but they are just not visible to you “right now” for one reason or another.
I know, these answers are not quite what you may be looking for. Let’s look a little bit deeper by starting our search for how many stars can I see at night.
Quiz – How many stars are in the sky, how well do you know them?
Let’s start by taking a short quiz (answers are just a scroll away, so don’t peek!):
Which star is the brightest in the sky?
Which star is the closest to us?
Which star is the first one discovered to have a planet supporting life?
Which stars have a solar system?
Scroll down a bit to see the answers to this first part of the quiz.
While we are waiting to scroll (and to keep the peekers from peeking!)… let’s talk a little bit about how professional astronomers count the stars.
Annie Jump Cannon – Harvard star counter extraordinaire
Annie Jump Cannon – Harvard University Image: Public Domain
Have you ever tried to count the stars? If you have, you probably gave up at some point because there were too many to keep track of.
In the early 1900s there was a tenacious Harvard astronomer named Annie Jump Cannon who didn’t give up counting! In fact, she counted so many stars that she almost lost track.
To keep everything organized she started categorizing them by their colors. She invented the spectral classification system – O,B,A,F,G,K,M – and personally classified over 350,000 stars!
Her method of categorizing stars is still in use today.
The answers to the star Quiz
The Sun compared to other stars. Image: Daniel Cummings
The number of stars in the sky is…
SPOILER ALERT! – Here are the answers to the quiz above!
The quiz is a trick quiz because the answers to are all “the Sun.” If we don’t include the Sun in this quiz then we get very different answers and the answers are:
Sirius is the brightest star (it is almost as bright as Jupiter),
Proxima Centauri is the closest star (it is 4.22 light years away),
No star’s exoplanets have yet been proven to support life – the Sun is the only star so far to host life.
There is only one solar system. The answer to question 4 is truly a trick question: “No other stars have a solar system… because only one star is named “Sol” … the Sun! The “Solar System” is named after Sol – the Sun. See: Sol+ar = Solar. Other stars would have planetary systems named after the star. For instance, there may be a “Sirius-ar system” that hosts a planet like Earth.
The Known Universe
Ok, now you are done with the quiz let’s get counting stars. Go outside, look up, start counting!
Artist’s conception of the known universe. Credit: Pablo Carlos Budassi via Wikipedia
Before, we start… There is one tiny problem with counting the number of stars. Even with the best tools humans have invented, we can only see a small part of the universe. Astronomers call this the “known universe” and it stretches about 14 billion years back through time in one direction, and (we assume) 14 billion years back in another direction.
Ten sextillion stars in the known universe
Astronomers have estimated the number of stars in the Milky Way galaxy as one hundred billion (this is a low estimate by the way). The Milky Way is an average galaxy. Astronomers estimate that there are one hundred billion galaxies in the known universe (another low estimate). The math to figure out the total number of stars in the universe is as simple as the result is mind-boggling!
Counting to one million is not like Counting to one billion
A million is way, way smaller than a billion.
It would take 11 days to count to one million if you counted one number per second without doing anything else. Counting to a billion at the same pace (one per second) would take you 32 years of continuous counting!
Count to 1 million at a rate of 1 per second = 11 days Count to 1 billion = 32 years
– Arithmetic
If you counted all the stars in the Milky Way at the rate of one per second, it would take you about 3200 years to finish counting our one galaxy’s set of stars.
How many stars in my sky now?
Ok. So, let’s get real. Now we know how many stars there may be in the entire known universe, let’s narrow the question.
Here is how we will narrow the question. Let’s create a set of ideal conditions that will guarantee we see as many stars as possible.
This is the set of almost ideal seeing conditions. This set of conditions would guarantee seeing the most number of stars that anyone could ever see with the naked eye:
I am on a tall mountain top with no trees looking up at the perfectly clear and calm sky right now. It is midnight. I can now see approximately half the stars in the sky. These near perfect seeing conditions make it so that I can see stars to magnitude 6 with my 20/20 vision.
Number of stars in the sky I see
According to David Haworth of Stargazing.net there are approximately 7400 magnitude 6 or brighter stars. Remember, we can only see half the sky at any time. So, cutting 7400 in half we get 3700 stars. 3700 stars is the greatest number of stars that any earth-bound human can ever see at any one moment without a telescope or other aides.
Stars in my sky right now
Now we know how many stars you might ever see at once, let’s look at how most people see the sky.
Most people cannot see 3700 stars because of serious light pollution, horizons full of trees and landforms, and atmosphere, lots of watery, moving atmosphere floating above us.
If you are lucky and go camping in a dark sky area you might be able to see about 2000 stars on a good night.
In a typical, light-polluted suburban sky you can see approximately 200-300 stars.
In the middle of a big city the number of stars goes down to about 12 to 20.
12 Stars.
12. That’s not a lot of stars. Maybe take a trip out of the city to see the stars?
Summary
You can see 3700 stars under perfect viewing conditions.
You can see about 2000 under excellent dark skies.
You can see about 300 stars in the suburbs.
You can see 12 stars under bright city lights.
References for how many stars
Stargazing.net – David Haworth compiled data on the number of stars at each magnitude and presented it in a nice table.
Sky and Telescope article – covers skyglow, light measuring technology, light maps, NELM and more. Authoritative.
Space.com article – how astronomers estimate the number of stars in the Milky Way.
National Geographic Kids – Mini article on light pollution with estimates on how many stars are visible from different places.
