M83

Milky Way Galaxy: A Spiral (Barred Spiral?) Galaxy

NGC 1232

It is difficult to deduce the appearance of the Milky Way because we happen to live inside of the Milky Way. We do believe, however, that the Milky Way is a spiral galaxy similar to M100, M51), and so on, despite the fact that, to us, the Milky Way appears to us like M104, Sombrero Galaxy in visible light. Our poor view of the Milky Way galaxy in visible light is due to the obscuration caused by the dust contained in the disk of our Galaxy. In light of this, how can we go about the study of the structure of the Milky Way galaxy?


Structure Tracers in Our Galaxy

Young stars ( OB stars, Pleiades star cluster) are bright and produce HII (ionized hydrogen) regions (Eagle Nebula). Ionized hydrogen atoms (HII) can produce radio emission. This is important because it allows us to see the HII regions (regions of recent star formation which concentrate to the spiral arms of our Galaxy) in our Galaxy. HII regions (and OB stars) are thus nice ways to trace out the spiral arms of our Galaxy.

At left is the spiral galaxy M33. The bright red knots are HII regions.

HI (neutral hydrogen) regions. An important fact is that neutral hydrogen atoms, can produce radio emission. In particular, HI atoms will produce radio emission with a wavelength of 21 cm. This is good because hydrogen is the most abundant element in the Universe. Although HI is pervasive in the disk of our Galaxy, it tends to clump in clouds which concentrate toward the spiral arms of our Galaxy. The HI clouds are thus useful ways to map out the structure of our Galaxy even in regions which do not contain large amounts of normal stars.

At left is a radio map (in 21 cm emission) of the Galaxy in HI emission.

Giant Molecular Clouds (star formation sites, Orion Molecular Cloud) generate micro-waves and infrared emission. Shorter in wavelength (when compared to radio emission) but longer in wavelength than visible light, micro-waves and infrared emission are useful probes of the structure of our Galaxy. It is found that Giant Molecular Clouds contain carbon monoxide (CO) molecules which emit radiation with a wavelength of 2.6 mm.

Cepheids (and also RR Lyrae stars) are objects which periodically vary in their power output (their luminosities). A remarkable result discovered by Henrietta Leavitt of the Harvard College Observatory was that there was well-defined relationship between the period of their pulsation and the intrinsic average brightness of the star (the period-luminosity relation). This allows Cepheids (and RR Lyraes) to be used to judge distances to far-off objects. In particular, this allowed Harlow Shapley to accurately map the positions of globular clusters and so determine the location of the Sun in the Milky Way Galaxy and to determine the size of the Milky Way Galaxy.


Rotation Curve of the Milky Way Galaxy

Beyond showing us what the Milky Way Galaxy looks like, we can also infer another interesting feature of the disk of our Galaxy from the above structure tracers; we can map out how material moves in the disk of our Galaxy. Using the Doppler shift, we are able to determine how fast objects and gas clouds move around the center of our Galaxy. We find the following rotation curve for the Milky Way Galaxy:

The point where the visible disk of the Galaxy ends (the disk as defined by the stars) is marked on the above figure. Interestingly, we can see gas beyond the end of the star-disk of our Galaxy. We use this information in the next section to infer an interesting fact about the mass and make-up of the Milky Way galaxy.


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