ch.17 Flashcards
(56 cards)
What is a galaxy?
An immense and relatively isolated cloud of hundreds of millions to hundreds of billions of stars, and vast clouds of interstellar gas.
Galaxies vary in shape, size, and content.
How do galaxies cluster?
Galaxies tend to cluster together and these clusters appear to be separating from each other in an expanding Universe.
The reason for this diversity in clustering is still unanswered.
What are the three main types of galaxies identified by Edwin Hubble?
- Spiral Galaxies
- Elliptical Galaxies
- Irregular Galaxies
Hubble also identified two sub-types: Barred Spirals and S0 Galaxies.
What distinguishes Spiral Galaxies?
Two or more arms winding out from the center, classified with letter S followed by a letter (a-d) indicating size and arm tightness.
The Milky Way is probably an SB galaxy.
What characterizes Elliptical Galaxies?
They have a smooth and featureless appearance and a generally elliptical shape, classified with letter E followed by a number (0-7) for flatness.
E0 is the most circular, while E7 is the most elongated.
What defines Irregular Galaxies?
They lack arms or uniform appearance, with stars and gas clouds scattered in random patches, classified as Irr.
These galaxies often exhibit a chaotic structure.
What is the interstellar content of Spiral Galaxies?
Approximately 15% by mass in the disk.
They contain a mix of Population I and Population II stars.
What types of stars are found in Elliptical Galaxies?
Only Population II stars, with blue stars being rare.
Interstellar content in Ellipticals is very low density.
What type of stars are common in Irregular Galaxies?
Blue stars.
They can contain as much as 50% by mass of interstellar content.
What is the Hubble Law?
A formula discovered by Edwin Hubble showing that recessional velocity (V) increases with distance (D) from the Milky Way.
The formula is V = H × D, where H is the Hubble constant.
What are standard candles in astronomy?
Objects like Cepheid variables, supergiant stars, planetary nebulas, and supernovas used to measure galaxy distances.
Their luminosity can be used to calculate distances.
What is a galaxy cluster?
Groupings of galaxies held together by their mutual gravity, typically several million light-years across.
Clusters can contain a handful to several thousand galaxies.
What is the Local Group?
A small cluster that includes the Milky Way and about 30 members, primarily dwarf galaxies.
The three largest members are M31, M33, and the Milky Way.
What is a supercluster?
A larger structure formed by gravitationally attracting groups of galaxy clusters.
The Local Group belongs to the Local Supercluster.
True or False: The most circular of the elliptical galaxies are classified as E7.
False.
E0 is the most circular, while E7 is the most elongated.
What type of galaxy is formed as a result of galaxy collisions and mergers?
Elliptical galaxy.
These mergers often leave little gas remaining in the system.
What is the significance of the rotation curve of a galaxy?
It is a graph of the orbital velocities at various distances from the center of the galaxy.
It helps in understanding the distribution of mass within the galaxy.
What is one piece of evidence for dark matter?
The rotation curves of most galaxies become flat at large distances from the center.
This indicates more mass is present than can be observed.
What is the likely outcome after the collision between two spiral galaxies?
Enhanced formation of new stars and formation of a larger galaxy.
Destruction of most stars is not typically expected.
True or False: Seyfert galaxies are bright elliptical galaxies.
False.
Seyfert galaxies are spiral galaxies whose nucleus is abnormally luminous.
What are blazars?
Quasars seen directly down the axis of their powerful jets.
They are a type of active galaxy.
What is the origin of synchrotron radiation in radio galaxies?
High speed electrons spiraling around the magnetic field lines.
This radiation is often observed in the lobes of radio galaxies.
What is the most likely origin of a massive black hole at the center of most galaxies?
A supernova explosion formed a black hole, which became more massive by accretion.
The black hole’s growth can also be influenced by the gravitational attraction of surrounding stars.
What is the accurate determination of star distances by the method of standard candles reliant on?
Accurate knowledge of the luminosity of the standard candle.
This knowledge is essential for distance calculations.
