Yellow Hypergiants

Yellow Hypergiants: A New Class of Star

Hypergiants are one of the newest classifications for stars that have appeared in the last few years. These stars are the most massive and most luminous stars know to exist, coming in three varieties: blue hypergiants (or LBVs), yellow hypergiants (YHGs), and red hypergiants (RHGs); a fourth classification, white hypergiants, are sometimes referred to, but it is not always recognized as a valid classification.

The classification was started because Astronomers thought that the luminous Supergiant category (Ia) encompassed too great a luminosity range and they thought that it should be broken up into two categories Ia and 0 (which is now sometimes shown as Ia⁺, Ia-0 or just 0 category). The new classifications corresponded to the old Luminous Supergiant category and the new Hypergiant category. You can still see a mixture of all three (Ia⁺, Ia-0, and 0) in astronomy literature, referring to this new class of star.

(NOTE: For the remainder of this article, I will refer to them as Ia-0 class stars.)

One of the rarest versions of hypergiants is the yellow hypergiant (YGH). They are thought to be post-red supergiants that are undergoing a blue-ward evolution on the HR diagram and encompass A Ia-0, F Ia-0, and G Ia-0 class stars (de Jager, 1998). This evolutionary process is still poorly understood and models are continuingly being developed & revised as more information becomes available. At one point, there were supposedly only seven (7) known in our galaxy.

I decided to write this article because of the inaccuracies that I found on the web, mostly generated by Wikipedia (and other web pages that used Wikipedia for their source information). Wikipedia for the longest time stated that there were only 7 known in our galaxy, but at the same time, they listed Westerlund 1 as a source to check. Well, I did check and what I discovered was more YHGs in the cluster, which were not counted in the original seven stated.

The literature shows that Westerlund 1 is a massive open star cluster that was first discovered in 1961, but its true nature was not uncovered until 2005 (West, 2005). At that time, it was found to be a super star cluster never before seen this close to earth. It is thought to be the most massive star cluster in the entire Local Group (the group of galaxies to which the Milky Way belongs) and it was of a type usually only seen in colliding galaxies. The literature search showed that there were at least 4 and possibly more YHGs (the latest count is now 6) in the cluster.

This discovery obviously doubles the number of YHGs in the galaxy—yet Wiki (and most other sources) still only counted 7. Obviously, no one bothered to check how many YHGs were in Westerlund 1 and then add that number to the “total” number in the galaxy. They did not complete the update of their information.

The other problem was that the sources only listed 3 YHGs (if they listed any at all) and did not tell you the names of the rest. Being a completist, I could not leave this one alone. I had to know every YHG known in our galaxy. Unfortunately, there is no complete list anywhere on the web. The only Academic document that I could find that might have this list was not free (you had to pay $34 for the privilege of reading it). I therefore started searching each and every Astronomy paper on the web that was free to examine. My search was long and hard, as most papers were listed multiple times in searches. But, finally, I now believe that I possess most of the names of the YHGs that reside in our galaxy (and four more that are in other galaxies).

The list is as follows:

Ø                Rho (ρ) Cassiopeiae (HD 224014)

Ø                HR 8752 (HD 217476)

Ø                IRC +10420

Ø                V382 Carinae (HD 96918)

Ø                V766 Centauri (HD (119796)

Ø                AFGL 2343 (HD 119796)

Ø                b Velorum (HD 179821)

Ø                6 Cassiopeiae (HD 223385)

Ø                Star 15 in RSGC1 (Star #15 in Red Supergiant Cluster 1, see Figer, 2006)

                     Star 49 of RSGC2 (Star # 49 in the Red Supergiant Cluster 2, see Davies, 2007)

From the Westerlund 1 cluster:

Ø                W4

Ø                W8a

Ø                W12a

W1          W16a

Ø                W32

Ø                W265

Possibly a YHG (but more likely a Red Hypergiant, or RHG):

Ø                RW Cephei

Extragalactic YHGs include the following:

Ø                Variable A (in the galaxy M33)

Ø                HD 33579 (in the Large Magellanic Cloud)

                    HD 269953 (in the Large Magellanic Cloud)

Ø                HD 7583 (in the Small Magellanic Cloud)

These stars have been collected in Figure 1, below. I will be updating the table as new information becomes available and as I read new astronomy papers on this subject.

Figure 1: Table of Yellow Hypergiants
(Click to view larger Image)

From this, there are at least 16 YHGs in our galaxy that we know of today. The astronomy paper on Massive Young Clusters (Figer, 2008), speculates that more massive clusters will be recognized in the future, which will bring new discoveries of YHGs.

The future looks “bright” for the study of YHGs.

References

Clark, J. S., Negueruela, I., Crowther, P. A., & Goodwin, S. P., 2005, A&A 434, 949

Davies B., Figer, D. F., Kudritzki, R.-P., MacKenty, J. W., Najarro, F., & Herrero, A., 2007, ApJ 671, 781

de Jager, C., 1998, A&AR 8, 39

Figer, D. F., MacKenty, J. W., Robberto, M., Smith, K., Najarro, F., Kudritzki, R.-P., & Herrero, A., 2006, ApJ 643, 1166

Figer, D. F. 2008, “Young Massive Clusters”, Proc. IAU Symposium No. 250, Massive Stars as Cosmic Engines, F. Bresolin, P.A. Crowther & J. Puls, eds.

Humphreys, R. M., Strecker, D. W., and Ney, E. P. 1971, ApJ 167, L35

Nieuwenhuijzen, H. and de Jager, C., 2000, A&A 353, 163

Oudmaijer, R. D., Davies, B., de Wit, W.-J., Patel, M. 2008, Arvix preprint arXiv:082315

Reddy, B. E., Hrivnak, B. J. 1999, AJ 117, 1837

West, (citation to be completed!)