bystander wrote:A Matter of Distance
ESA Hubble Picture of the Week | 2017 Apr 24
In space, being outshone is an occupational hazard. This NASA/ESA Hubble Space Telescope image captures a galaxy named NGC 7250
. Despite being remarkable in its own right — it has bright bursts of star formation
and recorded supernova explosions
— it blends into the background somewhat thanks to the gloriously bright star hogging the limelight next to it.
This bright object is a single and little-studied star named TYC 3203-450-1
, located in the constellation of Lacerta
), much closer than the much more distant galaxy. Only this way a normal star can outshine an entire galaxy, consisting of billions of stars. Astronomers studying distant objects call these stars “foreground stars” and they are often not very happy about them, as their bright light is contaminating the faint light from the more distant and interesting objects they actually want to study.
In this case TYC 3203-450-1 million times closer than NGC 7250 which lies over 45 million light-years away from us. Would the star be the same distance as NGC 7250, it would hardly be visible in this image.
It's a great-looking galaxy, obviously starbursting and quite dusty. But look at its nucleus, how faint and small it is! It isn't even obvious which extended whitish central point is the actual nucleus, although one of them is the likeliest candidate. But if a galaxy has such a tiny nucleus and such an unimpressive yellow bulge, it is certainly a light-weight and tiny galaxy that hasn't formed a lot of stars in the past. Now, however, it is making up for lost time!
And now look at that star, TYC 3203-450-1! It looks orange, and it is: its B-V is ~ 1.2 or 1.3. That is way redder than the B-V of the Sun, whose color index is 0.656 ± 0.005, and it is redder still than the galaxy, whose (dust-reddened) B-V is 0.640. Yet, a B-V of 1.2 or 1.3 isn't tremendously
red for a star, and it doesn't suggest an M-type star to me. Maybe an M0V star, the brightest and the least red of the M-type main sequence stars? Is that possible?
Fantastically, according to my software Guide, the visual magnitude of TYC 3203-450-1 is 10.933. That is incredibly similar to the visual magnitude of Proxima Centauri, the most nearby of all stars after the Sun, whose visual magnitude (Proxima's, not the Sun's) is 10.977. These two stars are almost exactly the same apparent brightness! Yes, but Proxima is way redder, with a B-V index of 1.8, and its spectral class is M5V.
And now consider the distance to these two stars, and their intrinsic brightnesses. The distance to Proxima is 4.227 ± 0.014 light-years, and the star's brightness is 0.000055 times the Sun! That's so faint that it's amazing! Yes, but if I read the ESA/Hubble caption right, the distance to TYC 3203-450-1 would be 45 light-years, pretty much ten times farther away than Proxima! What does that tell us about the intrinsic brightness of TYC 3203-450-1? Is it a hundred times brighter than Proxima? So would that make it around 0.0055 times the luminosity of the Sun? That still seems low to me. Is TYC 3203-450-1 an M-type main sequence star after all, in spite of its relatively non-red appearance? Is it a star of spectral class M0V?
I'd love to hear your opinions on this! But if no one likes to chime in, I still find the portrait of the star and the galaxy (and the accompanying caption) incredibly interesting.