Sat, 01/07/2017 - 20:16
Hi everyvody,
I am a non-specialist who have a question ...
I have a general question about star light fluctuation. Most of the fluctuations have a period of several years. However, I would like to know if the classical stars also have fluctuations in brightness at higher frequency, for example over periods of second or minute? These fluctuations, if they exist, should be of small amplitude.
I am talking about real fluctuation, that could be seen from space, and not fluctuations on earth because of atmospheric disturbance. I am not talking about phenomena related to exoplanets either.
Thank you!
Actually, most of the stars being observed by the AAVSO undergo changes in luminosity on time scales that are typically much less than one year. Speaking only of intrinsic fluctuations (i.e., due to changes in the star itself rather than its being eclipsed by an orbiting companion or orbiting debris including planets), there are many types of variables that manifest cycles of luminosity changes on time scales of days, weeks or months. These include, for example, Cepheids, RR Lyrae stars and Miras. As these stars expand and contract, they not only change brightness but color! In addition to these periodic variables, there are also many classes of irregular variables, some of which can undergo large changes in luminosity on time scales of hours or less. You can find lots of information on the various kinds of variable stars and the time scales of their variability under several of the Variable Stars pages on this site.
In addition to the foregoing - which is a ridiculously abbreviated overview of a rich subject - there are also stellar oscillations due to coherent acoustic waves propagating in the star interiors. These wave propagation phenomena organize according to the structure and symmetry of the host star, and produce very small scale changes in emitted radiation on time scales as small as fractions of an hour. Look up astroseismology for more information,
Thank you Stephen. I now have a better view of these variable stars.
I am interested in [0.1 - 20] Hz fluctuation, particulary non predictable fluctuation if it exist. Astroseismology seems to be a good way.
Do you think these oscillation can be measured by amateur telescopes?
As far as astroseismology is concerned, I think the answer is mostly No. The fluctuations in luminosity are so tiny as to be essentially masked by noise and seeing effects when observed from the earth's surface. The way the effects are observed is via high resolution spectroscopy, using Doppler shifts to infer the oscillation spectrum. Spectral estimation (in the sense of frequency domain signal processing) is then used to estimate the amplitudes associated with the various modes of oscillation.
The astroseismology web site (http://asteroseismology.org/) is a good source of material on the theoretical background with links to data and published papers.
If you have a truly big scope with a good spectrograph you might try your hand at Procyon, which exhibits solar-like oscillations.
Re "can fast fluctuations be meaured by amateur telescopes?", that obviously depends a bit on the definition of "amateur 'scopes" and the instrument that you're prepared to bring to the party. There is an interesting description by Richard Stanton in the 2015 SAS Proceedings, showing his fast-photometer on a 30-inch Dob He shows observations of flares on UV Ceti; one flare made the star double in brightness, over a rise-and-fall time of about 10 seconds.
You can download the Proceedings from the SAS website (free) at www.SocAstroSci.org, on the Publications tab.
Bob B.