Tag: star

BAFact Math: The Sun is 12 *trillion* times brighter than the faintest star you can see

By Phil Plait | August 28, 2012 10:00 am

[BAFacts are short, tweetable astronomy/space facts that I post every day. On some occasions, they wind up needing a bit of a mathematical explanation. The math is pretty easy, and it adds a lot of coolness, which I’m passing on to you! You’re welcome.]


Today’s BAFact: The Sun is 12 trillion times brighter than the faintest star you can see with your naked eye.

In yesterday’s BAFact, I showed how the Sun is about 400,000 times brighter than the full Moon – and I showed my math. That’s an amazing brightness difference, but while I was writing it I had to wonder: how much brighter is the Sun than the faintest star you can see?

The faintest stars visible to the naked eye have a magnitude of about 6. This depends on lots of stuff, like how dark the sky is, how good your eyesight is, and so on. Some people with excellent vision can see stars down to magnitude 7, and there are reports of a few extraordinary people who can see even fainter. But on a dark night, the average person can just barely see 6th magnitude stars.

Let’s use that number then. All we have to do is plug that into the equation I gave yesterday (and remembering that the Sun has a magnitude of -26.7):

Brightness ratio = 2.512(6 – (-26.7)) = 2.51232.7 = 12 trillion

Yegads! That’s 12,000,000,000,000 times brighter!

Now, to be fair, that’s not really the brightness range your eyes can detect. You can’t look right at the Sun easily or comfortably; it’s simply too bright. So the range of brightness your eye can see is probably smaller.

We can put a lower limit on it easily enough using the Moon. The Moon is the second brightest object in the sky, and we know we can look at that easily enough, so let’s do that math (the Moon’s magnitude is -12.7 when it’s full):

Brightness ratio = 2.512(6 – (-12.7)) = 2.51218.7 = 30 million

Wow. So you can comfortably see objects over a brightness range of 30 million. That’s impressive! The eye is a pretty cool little machine.

As an aside, your eye isn’t linear; it’s logarithmic (in reality, it’s more complicated than this, and I’m simplifying, but close enough). In other words, a star giving off twice as much light doesn’t look twice as bright as another. The way your eye responds to light squeezes down the scale, making it easier to see fainter and brighter objects at the same time.

So how faint do objects get? Ah, that’ll be tomorrow’s BAFact. Stay tuned!


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CATEGORIZED UNDER: Astronomy, BAFacts
MORE ABOUT: magnitudes, Moon, star, Sun

Astronomers see ANOTHER star ripped apart by a black hole!

By Phil Plait | May 2, 2012 2:03 pm

Last year, astronomers saw the violent death throes of a star as it was literally torn apart by a black hole (see here, and links within). And now, they’ve seen it again: observations across the electromagnetic spectrum caught another star that wandered too close to a supermassive black hole, and suffered the ultimate fate.

These observations show the before-and-after (left versus right) of the event. The top two are from GALEX, a satellite that observes the skies in the ultraviolet, and the bottom using Pan-STARRS1, a powerful telescope (located on which mountain, you ask? Why, Haleakala in Hawaii, of course) that scans the entire night sky looking for transients, things that change brightness.

The light from the star’s violent demise reached us in June of 2010. The event happened in the heart of a distant galaxy, 2.7 billion light years away. At the center of that galaxy is a black hole with millions of times the Sun’s mass, comparable to the black hole in the center of our own Milky Way galaxy. The star apparently orbited the black hole in an elliptical orbit. Over millions or billions of years, the star evolved, and turned into a red giant. Over time, its orbit tightened, and one day it got too close. The enormous tides of the black hole tore the star apart.

The flare happened when the stellar material spiraled into the hole. It formed a flattened disk right before the Ultimate Plunge, which got very hot and blasted out high-energy light — the ultraviolet light from this galaxy flared 350 times brighter than it was before! Some of the material from the star was also flung away into space. Astronomers put together a nifty video simulating what happened:

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A red nursery with a long smoke stack

By Phil Plait | April 27, 2012 6:48 am

Something like 6000 light years away, roughly toward the downtown area of our galaxy, lies NGC 6604, a tight cluster of young, massive, hot, bright stars. Just starting to shrug off the gas cloud of its birth, these stars emit a fierce light that makes the gas glow. When you point the 2.2 meter ESO/MPG telescope at this cluster what you get is startling beauty:


[Click to ennebulenate, or grab the cosmic 8600 x 8400 pixel version.]

NGC 6604 is the compact group of bright blue stars in the upper left. This whole complex of gas (called Sharpless 2-54) is about 200 – 250 light years across, making it rather huge! You’re only seeing a fraction of it here, though. It’s actually part of an even larger series of nebulae which include the more famous Eagle nebula (the Pillars of Creation) and the Omega nebula.

The image is a composite of pictures from different filters. Ultraviolet and blue filtered images were combined to make blue in this image; green filtered light is colored green, yellowish light from nitrogen is yellow, and the red is actually red from warm hydrogen. As you can see, hydrogen is plentiful in this area!

Also, see those odd diagonal features on the lower left? Those extend for a long way, well outside the frame here. That structure is called a "chimney", and may be 650 light years long! As stars are born, they can blow massive winds from their surfaces. This puts pressure on the surround gas, and if there’s a weak spot — where the gas is less dense, or if it’s near the edge of the cloud — the winds can push through. It’s not clear exactly how these form, or why they tend to be so straight. It’s suspected magnetic fields are involved, but that complicates things hugely. Still, the chimney in Sharpless 2-54 is the closest one known (of dozens), providing a nice clear view of it. If we ever do figure out the detail mechanics of chimneys, no doubt this one will play a role.

Image credit: ESO


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CATEGORIZED UNDER: Astronomy, Pretty pictures
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