Stars and their apparent magnitude: How many stars can you see?

It’s a big week for space nerds like me. The European Southern Observatory (ESO) announced that they found a special planet near Proxima Centauri, the closest star to Earth. Some news agencies are calling it a “second Earth”, which is a wee bit sensational.

A more restrained way of describing things is that the ESO has discovered an Earth-sized planet in Proxima Centauri’s habitable zone, meaning it can theoretically support liquid water and life. Either way, it’s a very exciting discovery.

How can the ESO discover distant planets? Really big telescopes. The ESO operates the Very Large Telescope (its actual name) and is currently building the European Extremely Large Telescope (also its actual name). These telescopes let us see deeper into space than we’ve ever been able to.

What can you see without a really big telescope?

How much of the night sky is invisible to us when we don’t have really big telescopes like the ESO? What if we had just a little help, like a pair or binoculars or a small telescope? How much more can see see with these technologies?

It turns out, a lot. With the unaided eye, we can only see a small portion of known stars. You know that feeling of infinity when you look up at a really dark night sky? You’re probably looking at only 2,500 stars. Most stars aren’t bright enough to actually see. Astronomers even have a unit of measure for the brightness of celestial objects: apparent magnitude, which is how bright something appears to us on Earth. There are two things you should know about magnitude:

  1. The lower the magnitude, the brighter the object. Polaris, the North Star, has a magnitude of about 2, which is pretty bright. Pluto, everyone’s favourite dwarf planet, has a magnitude of about 14, which is really faint. Magnitude can be negative, too, which means something is really bright. A full moon has a magnitude of -13 and the Sun a magnitude of -27.
  2. Magnitude is a logarithmic scale. For example, a magnitude-1 object is 2.5x brighter than a magnitude-2 object. Here’s a visualisation where the area of the circles correspond to the brightness of the object, with the smallest dot representing a magnitude of 14.

visual-magnitude

The stars we know about

Now that we know about magnitude, we can use it to look understand more about how we see stars. The most comprehensive catalogue of star data I could find is the HYG Database, a fantastic datase compiled by David Nash, an amateur astronomer, that’s composed of almost 120,000 stars. Here are the stars plotted by magnitude. Each “x” is a star.

apparent-magnitude-1

The steep slopes at the beginning and end indicate the few stars that are low-magnitude (less than 5) or very-high magnitude (greater than about 12). Most stars have a magnitude of between 5 and 12. There are a few individual outliers with magnitudes below 0 or above about 18.

Another way to look at this is with a histogram.

magnitude-histogram

This tells us largely the same story: most stars we know about have a magnitude between 5 and 12. We can’t see the outliers like in the graph above, but that’s not important right now.

What does it take to see these stars?

That depends mostly on two things: where you are and what technology you’re using to stargaze.

Let’s assume you’re like me: you live in a city and you’re just using your eyes. These two factors are limiting. First, cities tend to have a lot of light pollution, making it harder to see the night sky. Second, our eyes don’t provide any magnification.

Of the 120,000 stars in our dataset, here’s what you’ll see.

magnitude-histogram-urban

See that tiny sliver of yellow? That’s actually double what you’ll be able to see. Our dataset considers the whole sky, but when we’re stargazing, about half of these stars will be below the horizon.

Let’s say you live in a rural area or you’re on a camping trip or something — “dark sky” conditions. You still have just your eyes.

magnitude-histogram-dark-sky

That’s a lot more impressive. That’s the difference between blueish sky in the city and an inky black sky in the wilderness.

We’re still not seeing most stars, though. Even with dark skies, we only seeing 4% of known stars. Let’s buy set of decent binoculars.

magnitude-histogram-binoculars

Wow! What a difference. You can even see some of Jupiter’s moons with binoculars — the same ones Galileo discovered.

 

If binoculars aren’t cutting it, we can get a modest telescope.

magnitude-histogram-telescope

 

If you really want to up your game, you can use the Hubble Space Telescope. The Hubble can see everything in our star catalogue, plus a whole lot more: its visible light limit is a magnitude of 32.

magnitude-histogram-hubble

 

What does this tell us?

A couple things. First, the majority of known stars are invisible to us. As limitless as stars seem, there are a lot more hiding in the seemingly blank space in the sky.

Second, astronomy is surprisingly accessible. All you need is a pair of binoculars and you’ve got over 100,000 stars to look at, not mention all the cool stuff in our own solar system, like the Moon or most of the planets. You won’t be finding planets around other stars, but there’s still a lot to see in our own neighbourhood.

 


Source Information

The HYG Database is a combination of the Hipparcos Catalog, Yale Bright Star Catalog, and the Gliese Catalog. There are a lot of stars in this database — almost 120,000 — but it still represents a very small portion of stars in the universe. The Milky Way alone is estimated to have 100-400 billion stars. For our purposes, we’re going to assume that the HYG Database gives us a “directionally correct” idea of the stars we can observe with different technologies.

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