HD15558 – the most massive star in the Milky Way

Last session with LowSpec spectroscope was only a few hours long due to weather. But still it was enough to capture some spectra of Be type stars, but not only. I also pointed telescope to Melotte 15 open cluster and shot spectra of three young stars. These objects are often referred as YSO – Young Stellar Objects. Among them was blue giant HD15558 – the most massive star in the Milky Way!

Melotte 15 open cluster
Melotte 15 open cluster in Cassiopeia. Evostar 72, IMX224, 180x15s

HD15558 is a real monster. It is a member of multiple system. Its spectral type is quite early O4.5 III, so it is very hot – its surface temperature is 46800K. HD15558 is 152 times more massive than Sun, and it is 660,000 times more luminous than Sun! I cannot imagine the star that shines so much stronger than Sun, but there are much more luminous stars in the Universe.

Spectra described

And here are the spectra of three young stars annotated in the image above. HD15558 is the hottest among them – type O4.5 III. BD+60 501 has little bit later spectral type – O7 V. So it is less hot. And last one is BD+60 505 of type A1 IV. You may see how the star spectral type affects its spectrum. O type stars contains strong lines of both hydrogen and helium. The number in O type stars is defined by the ratio of He I and He II lines. Another O type specific spectrum feature are diffuse interstellar matter bands – here visible at 4430 and 5780A. These features are not originated in the star itself, but they appear in the spectra of O type stars. When you compare last spectrum (A type star) you notice, that there are no more helium lines. A type stars are too cold to have them. Another difference is the width of hydrogen absorption lines – much larger in A type stars. Also carbon and nitrogen lines are no more present in A type star, but you may see Mg line appears slowly at 4481A.

Melotte 15 stars spectra
Melotte 15 stars spectra

The spectra above are a little bit noisy, especially in the left part. QHY163M camera is not so sensitive in the violet part, and also I had not much time to capture the data, because the clouds were coming. Recording spectra is all about time and aperture, not much about optics quality and flat field of view. The more photons you capture, the better data you achieve 🙂

Clear skies!

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