Abstract
The Earth is orbiting around the Sun, which itself is orbiting around the center of the Milky Way. Our Milky Way, the Galaxy, is the only galaxy in which we are able to study astrophysical processes in detail. Therefore, our journey through extragalactic astronomy will begin in our home Galaxy, with which we first need to become familiar before we are ready to take off into the depths of the Universe. Knowing the properties of the Milky Way is indispensable for understanding other galaxies.
This is a preview of subscription content, log in via an institution to check access.
Access this chapter
Tax calculation will be finalised at checkout
Purchases are for personal use only
Notes
- 1.
The equatorial coordinates are defined by the direction of the Earth’s rotation axis and by the rotation of the Earth. The intersections of the Earth’s axis and the sphere define the northern and southern poles. The great circles on the sphere through these two poles, the meridians, are curves of constant right ascension α. Curves perpendicular to them and parallel to the projection of the Earth’s equator onto the sky are curves of constant declination δ, with the poles located at δ = ±90∘.
- 2.
In general, since the star also has a spatial velocity different from that of the Sun, the ellipse is superposed on a linear track on the sky; this linear motion is called proper motion and will be discussed below.
- 3.
To be precise, the Earth’s orbit is an ellipse, and one astronomical unit is its semi-major axis, being 1 AU = 1. 496 × 1013 cm.
- 4.
i.e., to the main sequence in a color-magnitude diagram in which absolute magnitudes are plotted.
- 5.
With what we have just learned we can readily answer the question of why the sky is blue and the setting Sun red.
- 6.
This notation scheme (Type Ia, Type II, and so on) is characteristic for phenomena that one wishes to classify upon discovery, but for which no physical interpretation is available at that time. Other examples are the spectral classes of stars which are not named in alphabetical order nor according to their mass on the main sequence; or the division of Seyfert galaxies into Type 1 and Type 2. Once such a notation is established, it often becomes permanent even if a later physical understanding of the phenomenon suggests a more meaningful classification.
- 7.
Pulsars are sources which show a very regular periodic radiation, most often seen at radio frequencies. Their periods lie in the range from ∼ 10−3 s (milli-second pulsars) to ∼ 5 s. Their pulse period is identified as the rotational period of the neutron star—an object with about one Solar mass and a radius of ∼ 10 km. The matter density in neutron stars is about the same as that in atomic nuclei.
- 8.
The name of a supernova is composed of the year of explosion, and a single capital letter or two lower case letters. The first detected supernova in a year gets the letter ‘A’, the second ‘B’ and so on; the 27th then obtains an ‘aa’, the 28th an ‘ab’ etc. Hence, SN 1987A was the first one discovered in 1987.
- 9.
Hii-regions are nearly spherical regions of fully ionized hydrogen (thus the name Hii region) surrounding a young hot star which photoionizes the gas. They emit strong emission lines of which the Balmer lines of hydrogen are strongest.
- 10.
These energies should be compared with those reached in particle accelerators: the LHC at CERN reaches ∼ 10 TeV = 1013 eV. Hence, cosmic accelerators are much more efficient than man-made machines.
- 11.
Shock fronts are surfaces in a gas flow where the parameters of state for the gas, such as pressure, density, and temperature, change discontinuously. The standard example for a shock front is the bang in an explosion, where a spherical shock wave propagates outwards from the point of explosion. Another example is the sonic boom caused, for example, by airplanes that move at a speed exceeding the velocity of sound. Such shock fronts are solutions of the hydrodynamic equations. They occur frequently in astrophysics, e.g., in explosion phenomena such as supernovae or in rapid (i.e., supersonic) flows such as those we will discuss in the context of AGNs.
- 12.
The Pierre Auger Observatory in Argentina combines 1600 surface detectors for the detection of particles from air showers, generated by cosmic rays hitting the atmosphere, with 24 optical telescopes measuring the optical light produced by these air showers. The detectors are spread over an area of 3000 km2, with a spacing between detectors of 1. 5 km, small enough to resolve the structure of air showers which is needed to determine the direction of the incoming cosmic ray. Starting regular observations in 2004, Auger has already led to breakthroughs in cosmic ray research.
- 13.
In addition to the two-photon annihilation, there is also an annihilation channel in which three photons are produced; the corresponding radiation forms a continuum spectrum, i.e., no spectral lines.
- 14.
The determinant in (2.86) is a generalization of the derivative in one spatial dimension to higher dimensional mappings. Consider a scalar mapping y = y(x); through this mapping, a ‘small’ interval Δ x is mapped onto a small interval Δ y, where Δ y ≈ (dy∕dx) Δ x. The Jacobian determinant occurring in (2.86) generalizes this result to a two-dimensional mapping from the lens plane to the source plane.
- 15.
- 16.
These parallax events in addition prove that the Earth is in fact orbiting around the Sun—even though this is not really a new insight….
- 17.
Masers are regions of stimulated non-thermal emission which show a very high surface brightness. The maser phenomenon is similar to that of lasers, except that the former radiate in the microwave regime of the spectrum. Masers are sometimes found in the atmospheres of active stars.
- 18.
One problem in the combined analysis of data taken in different wavelength bands is that astrometry in each individual wavelength band can be performed with a very high precision—e.g., individually in the radio and the IR band—however, the relative astrometry between these bands is less well known. To stack maps of different wavelengths precisely ‘on top of each other’, knowledge of exact relative astrometry is essential. This can be gained if a population of compact sources exists that is observable in both wavelength domains and for which accurate positions can be measured.
Author information
Authors and Affiliations
Rights and permissions
Copyright information
© 2015 Springer-Verlag Berlin Heidelberg
About this chapter
Cite this chapter
Schneider, P. (2015). The Milky Way as a galaxy. In: Extragalactic Astronomy and Cosmology. Springer, Berlin, Heidelberg. https://doi.org/10.1007/978-3-642-54083-7_2
Download citation
DOI: https://doi.org/10.1007/978-3-642-54083-7_2
Published:
Publisher Name: Springer, Berlin, Heidelberg
Print ISBN: 978-3-642-54082-0
Online ISBN: 978-3-642-54083-7
eBook Packages: Physics and AstronomyPhysics and Astronomy (R0)