APOD: Globular Star Cluster 47 Tuc (2015 May 19)

[Ann]

According to http://en.wikipedia.org/wiki/Apparent_m ... al_objects on Sedna at aphelion our Sun has an Apparent Magnitude of -11.2, far brighter then Venus at ~-4. And that is at 936 AU. So, yeah...lotsa stars in the day sky.

Bear in mind that our Sun is pretty bright. You don't think so? But it is. Most stars are red dwarfs, and they are much fainter than the Sun. Consider Proxima Centauri, the nearest star after the Sun - it is 18,000 times fainter than the Sun! Proxima may or may be gravitationally bound to Alpha Centauri A and B, two sunlike stars. The distance from the tight pair to Proxima is at least 8,500 AU. But from Alpha Centauri A and B, faint but nearby Proxima would be a fourth magnitude star. That is magnitude +4, not -4, like Venus. Read about Alpha, Beta and Proxima Centauri here.

In a globular cluster like 47 Tuc, most stars are likely to be red dwarfs, much fainter than the Sun. They could come quite close to an observer and still appear pretty faint. If the night sky on a planet inside a globular cluster contained as many bright stars as alter-ego said, the stars would either have to be intrinsically at least moderately bright (like the Sun), or else incredibly nearby.

Ann

[neufer]

<<In a globular cluster like 47 Tuc, most stars are likely to be red dwarfs, much fainter than the Sun. They could come quite close to an observer and still appear pretty faint. If the night sky on a planet inside a globular cluster contained as many bright stars as alter-ego said, the stars would either have to be intrinsically at least moderately bright (like the Sun), or else incredibly nearby.

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<<Looking like glittering jewels, the stars in this Hubble Space Telescope image at left are part of the ancient globular star cluster NGC 6397. Scattered among these brilliant stars are extremely faint stars. Hubble's Advanced Camera for Surveys has taken a census of the cluster stars, uncovering the faintest stars ever seen in a globular cluster. The Advanced Camera found the faintest red dwarf stars (26th magnitude), which are cooler and much lower in mass than our Sun, and the dimmest white dwarfs (28th magnitude), the burned-out relics of normal stars.

The image at upper right pinpoints one of the dim white dwarfs (the blue dot within the blue circle) seen by Hubble. The white dwarf has been cooling for billions of years.

The image at lower right shows the faintest red dwarf star (the red dot within the red circle) spied by Hubble.

The images were taken with visual and red filters. NGC 6397, one of the closest globular clusters to Earth, is 8,500 light-years away in the southern constellation Ara. The data for these images were obtained in March and April 2005.>>

http://hubblesite.org/newscenter/archiv ... 6/37/full/

[MarkBour]

...
In detail, I attempted to answer this question last year for Omega Centauri. ... The results were interesting and impressive. ... Imagine looking up and seeing visible stellar densities exceeding the Pleiades everywhere! ...

Thanks, alter-ego. I would love to have someone perform this simulation carefully as you did and put it on at a planetarium. That would be awesome.
It'd also be cool to view it from the observation deck of Chris' super-fast spaceship.