guenthert wrote: ↑Mon Apr 27, 2020 4:38 am
This assumes that one can spatially resolve those flashlight, err, stars. Afaik, the Andromeda galaxy is thought to be approximately as large as the local one and contains about as many stars, i.e. about 10e8. It spans about the same area as the moon on the night sky, hence with current technology, never mind the one available to Hubble, it's impossible to resolve a single star.
Or a dozen clustered together or a hundred. What's the resolution limit? 1000 stars, a million? So if there is a variable star changing its luminescence periodically by a generous 1000%, in a cluster of 1000 stars, the total brightness will change only 1%. Hubble was able to detect that on photo plates and able to distinguish it from dimming caused by turbulence in the air?
Thanks for all the answers, but there's something I'm still missing.
You are very wrong indeed if you think it is impossible for the Hubble Space Telescope to resolve individual stars in the Andromeda Galaxy. Here
you can see a tiny field in Andromeda that Hubble has resolved down to stars certainly no brighter than Sirius, and possibly down to stars even fainter than the Sun.
More importantly, though, we don't need Hubble in order to resolve bright stars in Andromeda. Take a look at the picture at right, taken by amateur Jason Guenzel, also known as u/TheVastReaches. On this page
, he said this about his picture,
My extreme close-up portrait of the Andromeda Galaxy. Even amateur gear can reveal individual blue supergiant stars that are 2.5 million light years away.
Edwin Hubble resolved a likely Cepheid variable
in the Andromeda Galaxy back in 1923. Cepheids are bright, much brighter than the Sun. Delta Cephei
, the best-known Cepheid and the star that gave this class of stars its name, is probably about ~2,000 times brighter than the Sun, and Cepheids may possibly be as bright as 100,000 times solar. Such stars would certainly have been within reach for Edwin Hubble and the 100-inch (2.5 meter) Hooker Telescope
at Mount Wilson Observatory that he used to observe the Andromeda Galaxy.
As for the Hubble Space Telescope, one of its first missions was to search for Cepheids in galaxy Messier 100 (M100) in the Virgo Cluster in order to determine the distance to M100. The distance to M100 based on the luminosity of its Cepheids was determined to be around 56 million light-years, some ~25 times farther away than the Andromeda Galaxy. You can read about it here
In the long accompanying text, it is said that the Hubble constant, the rate of the expansion of the Universe, is 80, which would imply that the Universe is expanding at the rate of 80 km/sec per megaparsec. This expansion rate has now been revised down to ~70 km/sec per megaparsec. Note that the Hubble Telescope has not only been serviced but also upgraded since its pictures of Cepheids in M100 were taken in the 1990s.