Alnitak and Mintaka are two bright stars located at the ends of Orion’s Belt. Both belongs to rare spectral class O, that comprises the hottest and massive but also shortest living stars of the universe. At the end of their lives O-class stars will end in supernova explosion. O type stars live very short and quick, so in the Milky Way there are only about 20,000 representatives of this class. I have chosen Alnitak and Mintaka to record one of my first spectra using LowSpec spectroscope.

Orion's Belt
Orion’s Belt with Alnitak (top right) and Mintaka (bottom right)

What is spectrum anyway? It may be defined in several ways, but the visual spectrum that astronomers work with is the image obtained as a result of distribution of the light into the components with different wavelengths. Spectrum may contain emission and/or absorption lines – and that lines – their positions, shapes carry almost all the information we have about the Universe.

The spectra below have removed continuum, so the lines can be clearly identified. O class stars radiate most of the energy in UV range, so the actual raw spectrum would show large values in the blue range.

Alnitak and Mintaka spectra
Alnitak and Mintaka spectra

Alnitak and Mintaka are quite similar stars. Both are of O9.5 type, but Alnitak is supergiant and Mintaka is luminous giant. Surface temperature of these stars is about 30,000K. When compared to the Sun, Alnitak radiates about 100,000 times more energy. And in the same time it is “only” 20 times larger.

Both stars spectra are similar. Their contain absorption lines of helium He I and He II, and ionised elements like C III, C IV, N III, O III. But they also sometimes show emission lines of helium. In the Alnitak spectrum there is emission C III line at 5696A. This line is typical for Wolf-Rayet stars. And the H alpha line of Alnitak spectrum shows typical P Cygni profile. Emission part is shifted to red, and absorption part is shifted to blue. This is always an indication for radially ejected matter by the star.

Alnitak P Cygni profile
Alnitak P Cygni profile

The wavelength shift here is 6.3A, and according to Doppler law this yields a gas expansion velocity 287km/s.

Deep and wide absorption lines marked as O2 and H2O originate from oxygen and water in Earth atmoshpere.

Clear skies!

Technical data:

Date:       2020-01-16
Location:   Nieborowice, Poland
Telescope:  Meade ACF 10"
Instrument: LowSpec spectroscope, 600 lpmm
Camera:     QHY163M, gain 100
Mount:      SW EQ6
Guiding:    ASI290MM
Exposure:   Alnitak 10x30s, Mintaka 10x30s
Conditions: seeing average, transparency average