General

# How many stars?

As I continue to ponder the spooky character of quantum physics (the 10^-n reality), this May 17, 2017,  Space.com article addresses a question at the other end of the cosmic scale (the 10^n reality): How Many Stars Are In The Universe?

So how do astronomers figure out how many stars are in the universe? The first sticky part is trying to define what “universe” means, said David Kornreich, an assistant professor at Ithaca College in New York State. He was the founder of the “Ask An Astronomer” service at Cornell University.

Do a rough back-of-the-envelope calculation (or use the “back of a napkin”) — simple, yes?

• Consider the current model of the observable universe (which “appears to go back in time by about 13.7 billion light-years”).
• Estimate the number of galaxies.
• Estimate the average number of stars in a galaxy.

But each one of those factors is tricky, as discussed in the article. Regardless, even being conservative results in a mind boggling number.

Kornreich used a very rough estimate of 10 trillion galaxies in the universe. Multiplying that by the Milky Way’s estimated 100 billion stars results in a large number indeed: 1,000,000,000,000,000,000,000,000 stars, or a “1” with 24 zeros after it. Kornreich emphasized that number is likely a gross underestimation, as more detailed looks at the universe will show even more galaxies.

So, survey says? Sky surveys are interesting.

Wiki:

Recent estimates of the number of galaxies in the observable universe range from 200 billion (2×1011) to 2 trillion (2×1012) or more, containing more stars than all the grains of sand on planet Earth.

## 3 thoughts on “How many stars?”

1. John Healy says:

Speaking of the observable universe, this Space.com article “Curious Kids: Does Space Go on Forever?” explores that question.

Space probably does go on forever, but the truth is we don’t know. Not yet anyway. That’s what makes this a great question, because science is all about finding answers to things we don’t know yet.

Not that long ago, people thought that when they looked up at the night sky, they were seeing all of space. That was until Edwin Hubble came along. He was an American astronomer and what he found out was so amazing that NASA named the famous Hubble Space Telescope after him.

Credit: Tanya Hill, Museum Victoria – May 27, 2017

This is an article from Curious Kids, a new series aimed at children. The Conversation is asking kids to send in questions they’d like an expert to answer. All questions are welcome – serious, weird or wacky! This article was originally published at The Conversation. The publication contributed the article to Space.com’s Expert Voices: Op-Ed & Insights.

2. John Healy says:

On June 12, 2017, two interesting articles were posted on Space.com:

Milky Way Galaxy: Facts About Our Galactic Home

The Milky Way contains over 200 billion stars, and enough dust and gas to make billions more. … As late as the 1920s, astronomers thought all of the stars in the universe were contained inside of the Milky Way. It wasn’t until Edwin Hubble discovered a special star known as a Cepheid variable, which allowed him to precisely measure distances, that astronomers realized that the fuzzy patches once classified as nebula were actually separate galaxies.

What Is the Big Bang Theory?

The Big Bang Theory is the leading explanation about how the universe began. At its simplest, it says the universe as we know it started with a small singularity, then inflated over the next 13.8 billion years to the cosmos that we know today.

Examining the CMB also gives astronomers clues as to the composition of the universe. Researchers think most of the cosmos is made up of matter and energy that cannot be “sensed” with conventional instruments, leading to the names dark matter and dark energy. Only 5 percent of the universe is made up of matter such as planets, stars and galaxies.

3. John Healy says:

How many stars are there in the universe? How many black holes? Who cares? … what confidence is there in any estimate anyway?

Well, The Dark Energy Survey explores this matter.

• Big Think > “There are 40 quintillion black holes in our Universe” by Ethan Siegel (October 30, 2023) – LIGO and Virgo have detected scores of black holes, providing us with our first quasi-census.

By combining information about stars, black holes, and stellar and cosmic evolution all together, astronomers have the first robust estimate for black holes in the Universe: 40 quintillion. It’s more than almost anyone expected.

Key points

All told, astrophysicists conclude 40 quintillion (4 × 10^19) black holes exist within today’s Universe.

That equates to 1-2% of all stars eventually forming black holes: higher than all prior estimates.

If confirmed, this implies black holes comprise 0.04% of the cosmic energy budget.

References

1. Black holes evidently abound in the center of our Milky Way according to this NPR article: “Center of the Milky Way Has Thousands of Black Holes, Study Shows” (April 4, 2018).