Click here for more on the annular eclipse of the sun – China to Texas – on May 20 or May 21. Image Credit: Sancho Panza on Flickr
The star Deneb – visible by mid-evening every May – is one of the most distant of the bright stars. When you gaze at this star, you are gazing across a great distance of space. The exact distance to Deneb is not known for certain, with estimates ranging from about 1,425 light-years to perhaps as much as 7,000 light-years. EarthSky veteran sky blogger Larry Sessions said in his post about Deneb:
The best estimates likely are those obtained by the Hipparcos Space Astrometry Mission in the 1990s. A simple calculation from initial Hipparcos data gives the figure of 3,230 light-years, whereas the refined data yield just over 1,400 light-years. At any of these estimates distances, Deneb is one of the farthest stars the unaided human eye can see. It is so far, that the light that reaches the Earth today started on its journey well more than 1,000 years ago.
Why don’t astronomers know the distance to Deneb exactly, and why are there different estimates for the star’s distance? The answer is that science is not a body of facts. It’s a process. Different astronomers or teams of astronomers try to improve on published distance estimates to the stars, and their various estimates are then published and passed along.
Deneb is too far away for its distance to be measured by the only direct method – stellar parallax. The distance to stars within a few hundred light-years of the solar system can be determined fairly accurately by parallax. The basic principle of parallax you can demonstrate to yourself by holding a finger in front of your nose and gazing at it with one eye closed, then the other eye closed. When you do this, you see your finger appear to jump from side to side with respect to background objects. If you hold your finger farther from your nose, it’ll appear to jump a smaller distance.
As Earth orbits the sun, astronomers can measure the parallax of the nearer stars against the more distant starry background, first from one side of Earth’s orbit and then – six months later – from the opposite side. Measuring stellar distances directly by parallax (trigonometry) only works for the nearer stars, however. For more, read Wikipedia’s parallax page.
Delta Cephei, prototype of Cepheid variable stars
Indirect means – which may be subject to errors – must be employed to estimate the distances to the more distant stars, like Deneb. Deneb’s given distance represents an educational guess, not a certainty. But there is little doubt that Deneb is one of the most distant stars that you can easily see with the unaided eye.
Deneb is one the three brilliant stars in the famous Summer Triangle asterism, which you’ll see over the east-northeast horizon by mid to late evening tonight. An asterism is a recognizable group of stars that isn’t a constellation. At our mid-northern latitudes, Deneb will light up the evening sky from now till the end of the year.
Deneb: Among most distant stars visible
So, if a distant star in our Milky Way such as Deneb exploded today, it would take hundreds or thousands of years for people to see it on Earth? How about our Sun? If it exploded today, how long would it take for us to see it? Would we see it explode first, or would we be exterminated before even seeing the explosion? (I mean what travels faster, light or ignited particles coming from a star’s explosion). Just curious, thanks in advance for any input!
Hi Bibi,
Light is the fastest moving stuff in the universe. So if the sun exploded, we would see the explosion 8 minutes later, because the sun is about 8 light-minutes away. Not sure how long the explosion itself would take to reach us. I do know that when storms on the sun send gusts of charged particles into space (CMEs), it takes them several days to get here.
Best,
Deborah
Thanks a lot Deborah. I didn’t know light was the fastest moving stuff in the universe, it’s cool to learn something new!
Shame on your school teachers Bibi! Or maybe you weren’t paying attention. Either way, I’m glad you know now, the speed of light is 186,000 miles per SECOND. Watch “The Universe” on the Science channel to learn more. Rick.
Rick neither me or my school teachers are ignorants. I already knew what the speed of light is. But I distinctly remember teachers always saying “light travels way faster than sound” and giving typical examples –lightning vs. thunderstorms– but they never mentioned “light travels faster than EVERYTHING ELSE in the universe”, I’m 100% sure of that. Thanks for trying to help anyways, although your comment sounds more like a put-down.
What travels faster … human imagination…
So true.
Deneb has a magnitude of 1.25, it’s about 1 kpc. However, it has an absolute magnitude of -7. So, if Deneb were the same distance from Earth to Sirius it would exceed the brightness of the full moon, or am I wrong?
Hi Filipe.
Since Deneb’s distance is so uncertain, so is its absolute magnitude. Absolute magnitude refers to how bright the star would look to the eye at 32.6 light-years away. I see that Guy Ottewell in his The Astronomical Companion lists Deneb’s absolute magnitude at -8.7.
Let me try to figure . . .
Sirius distance is 8.6 light-years away. At that distance, a star’s apparent magnitude would be about 14.37 times brighter than its absolute magnitude. That much brighter (14.37 times) would be an increase in magnitude of about 2.82.
So given an absolute magnitude of -8.7, Deneb would shine at an apparent magnitude of -11.52 at Sirius’ distance. The full moon’s apparent magnitude is -12.7 on the average. So I think the full moon would probably outshine Deneb, even if this star were at Sirius’ distance away.
Bruce
I hear very short wave. And often, I hear strange sounds, they’re periodic and repeated for several days (at the same time, same frequency). These sounds come from space?
Filipe,
Possibly, but I really don’t know. You may want to read about Karl Jansky, the fellow who discovered cosmic radio waves at http://www.nrao.edu/whatisra/hist_jansky.shtml
Bruce
Excuse me. I want say “I hear short wave”.
If you are a linux user my husband has just written a program on C++, soon to be launched, which will track the position of any star from thousands of years ago to thousands of years in the future. So if you wanted to know what the night sky looked like when Jesus was born or what Hannibal of Carthage saw as he marched to the Battle of Cannae in August 216 BC, check out his program. The program is called GETAFIX (get it) and please read about it here: http://nonsensegirl.wordpress.com/2012/05/17/getafix-astronomy-star-plotting-program-linux/
He has also used the free to download data from Hipparcos but used only the brightest star data.
Hope you visit the site for a gander. Cheers