Re: [guide-user] Not GSC star, but Tycho-2

Bill J Gray Jan 26, 2004

You're right; there is a slightly odd problem here.

Almost always, the Tycho-2 ID is the same as the GSC ID. Quite
wisely (in my opinion), the creators of Tycho and Tycho-2 did not
want to generate a completely new naming system. So a star such
as "GSC 1234 5678" became "TYC 1234 5678". I saw no good reason
(at the time) to change "GSC" to "TYC".

But some stars in Tycho and Tycho-2 were absent in GSC. For
these, new numbers were added. In GSC zone 722, there were 1195
stars already in the GSC catalog. So the stars added in Tycho
and Tycho-2 have numbers such as "722 1196", "722 1197", and so
on. Thus, the star you mention: "722 1203".

In many cases, the star was actually a close binary. So you
might have "722 1203 1" and "722 1203 2" (and, in rare cases,
"722 1203 3".) Unless the star really _is_ a binary, though,
it is usually written as "TYC 722 1203". Guide follows this
rule; unless you see that third number, you know that the
object was detected as a single star.

The rule Guide _should_ use is this: if the star is in the
Tycho or Tycho-2 catalog, then it should be described as
"TYC n1 n2" or "TYC n1 n2 n3". Otherwise, it should be
described as "GSC n1 n2". I will see if I can fix this...
at the very least, it should be fixed in the dialog shown
when you click on a star!

Hope this helps more than it confuses.

-- Bill