by Ann » Fri Aug 29, 2014 5:36 pm
Mark Bour wrote:
I have a basic question about what I see in many APODs. If you look at the small excerpt here, for example, the stars appear to come in many sizes and levels of brightness. Naturally, my brain wants to put the larger and brighter ones as closer (I assume that strategy is generally correct, but would result in occasional mistakes.)
Take a look at
this picture. The arrow points at the star that is closer to us than any other star except the Sun,
Proxima Centauri. This red dwarf at about 4 light-years away is about one part in 15,000 as bright as the Sun in visual light.
Let's return to
the picture again. The bright yellowish star on the left is Alpha Centauri, a double star made up of one star about one and a half times brighter than the Sun and another one about half as bright as the Sun (if I remember correctly). Apart from Proxima Centauri, the tiny red dwarf, the two Sun-like stars of Alpha Centauri are the nearest stars to us (apart from the Sun) in the universe, about four light-years away.
But take a look at the bright bluish star on the right in the picture. It looks
almost as bright as Alpha Centauri. The blue star is Hadar, Beta Centauri. It is
almost 400 light-years away,
almost a hundred times as far away as Alpha Centauri, and it is more than 3,000 times as bright as Alpha Centauri in visual light. Its total light output, including ultraviolet light, is much higher than that.
I have read somewhere that 99% of the stars that we can easily see with the naked eye in the sky are intrinsically brighter than the Sun. But the Sun may be brighter than 95% of all the stars in the Milky Way because of the profusion in our galaxy of little red dwarfs like Proxima Centauri.
Go figure.
Ann
[quote]Mark Bour wrote:
I have a basic question about what I see in many APODs. If you look at the small excerpt here, for example, the stars appear to come in many sizes and levels of brightness. Naturally, my brain wants to put the larger and brighter ones as closer (I assume that strategy is generally correct, but would result in occasional mistakes.)[/quote]
Take a look at [url=http://www.astrosurf.com/lorenzi/images/proxima_300_28_10m_p400f.jpg]this picture[/url]. The arrow points at the star that is closer to us than any other star except the Sun, [url=http://en.wikipedia.org/wiki/Proxima_Centauri]Proxima Centauri[/url]. This red dwarf at about 4 light-years away is about one part in 15,000 as bright as the Sun in visual light.
Let's return to [url=http://www.astrosurf.com/lorenzi/images/proxima_300_28_10m_p400f.jpg]the picture[/url] again. The bright yellowish star on the left is Alpha Centauri, a double star made up of one star about one and a half times brighter than the Sun and another one about half as bright as the Sun (if I remember correctly). Apart from Proxima Centauri, the tiny red dwarf, the two Sun-like stars of Alpha Centauri are the nearest stars to us (apart from the Sun) in the universe, about four light-years away.
But take a look at the bright bluish star on the right in the picture. It looks [i]almost[/i] as bright as Alpha Centauri. The blue star is Hadar, Beta Centauri. It is [i]almost[/i] 400 light-years away, [i]almost[/i] a hundred times as far away as Alpha Centauri, and it is more than 3,000 times as bright as Alpha Centauri in visual light. Its total light output, including ultraviolet light, is much higher than that.
I have read somewhere that 99% of the stars that we can easily see with the naked eye in the sky are intrinsically brighter than the Sun. But the Sun may be brighter than 95% of all the stars in the Milky Way because of the profusion in our galaxy of little red dwarfs like Proxima Centauri.
Go figure.
Ann