Spectral Type - variable stars

[Guest]

I was trying to find my way around the GCVS catalogue and I picked a star rr Lyr. It gave the spectral type as A5.0 - F7.0. In SIMBAD it gave the spectral type as A8 - F7 E.  So I looked up spectral types and found a website which had a table of spectral types. The table was first divided into different categories - Main sequence, Giants, Supergiants. Then within each category it listed the spectral types based on temperature. My question is - what do the two identifiers mean. I can see for a main sequence star A5 means a temperature of 8310K and F7 means a temperature of 6400K. rr Lyr is a variable star so does it mean that the spectral type varies between A5 and F7 over its cycle?

I started looking at this because I'm interested in variable stars but I'm using a DSLR at present. AAVSO suggests that with a DSLR you should pick stars which are not too red and not too blue and gives a range of suitable B-V magnitudes. So I was trying to find the B-V magnitude for this star. It didn't give that information but it did give the spectral type as mentioned above and I think that gives me the information I want.

Cheers

Steve

 

[andrew s]

Yes the spectral type varies as the star goes through its cycle. rr Lyr is the exemplar for a class of variables. You can find lots of info in the web about it and them.

Try the astro database SIMBAD to look up the star for its V and B magnitudes.

Regards Andrew 

Edited June 20, 2021 by andrew s

[Guest]

Thanks Andrew. I use SIMBAD already. When I looked up rr lyr it only gives the B magnitude- most stars it gives both.  Presumably since its spectral type varies through its cycle then if the B magnitude is constant then the V magnitude must vary quite a bit. AAVSO suggests that for DSLR observer the B-V value should be between about 0.3 and 1 with an ideal value around 0.7.  I have a list of stars I'm interested in and I want to eliminate the ones which are two red or too blue. 

Cheers

Steve

 

[andrew s]

Try looking up in the aavso data base. I am sure it will have both V and B estimates but expect it  to vary. In fact for such stars V-B varies considerably. I would not worry about it though just give it a go.

Wikipedia gives 0.181 for v-b

Regards Andrew

Edited June 20, 2021 by andrew s