Zoo of Galaxies 1) Explain the properties of spiral, elliptical, and irregular g

Question

Zoo of Galaxies

1) Explain the properties of spiral, elliptical, and irregular galaxies. Include the following:

What is the percentage each of spiral, elliptical and irregular galaxies (with respect to the total number of galaxies)?

Which type (spiral, elliptical or irregular) of galaxy has a well-pronounced core?

2) Explain the colors of spiral, elliptical, and irregular galaxies, including the following topics:

Do you see any blue elliptical galaxies? If not, why?

What color do active or interacting galaxies show? Explain why.

What is the percentage of red and blue irregular galaxies out of the total number of galaxies?

3) Identify how many of the observed galaxies are similar to our Milky Way galaxy:

What features do you use to compare them?

4) Describe the types of stars in spiral, elliptical, and irregular galaxies.

5) Describe the evolution of our understanding of the structure and extent of the universe.

Explanation / Answer

There are three main types of galaxies: Elliptical, Spiral, and Irregular.

1) Elliptical galaxies are shaped like a spheriod, or elongated sphere. In the sky, where we can only see two of their three dimensions, these galaxies look like elliptical, or oval, shaped disks. The light is smooth, with the surface brightness decreasing as you go farther out from the center. Elliptical galaxies are given a classification that corresponds to their elongation from a perfect circle, otherwise known as their ellipticity. The larger the number, the more elliptical the galaxy is. So, for example a galaxy of classification of E0 appears to be perfectly circular, while a classification of E7 is very flattened. The elliptical scale varies from E0 to E7. Elliptical galaxies have no particular axis of rotation.

2)Spiral galaxies have three main components: a bulge, disk, and halo. The bulge is a spherical structure found in the center of the galaxy. This feature mostly contains older stars. The disk is made up of dust, gas, and younger stars. The disk forms arm structures. Our Sun is located in an arm of our galaxy, the Milky Way. The halo of a galaxy is a loose, spherical structure located around the bulge and some of the disk. The halo contains old clusters of stars, known as globular clusters.

Spiral galaxies are classified into two groups, ordinary and barred. The ordinary group is designated by S or SA, and the barred group by SB. In normal spirals (as seen at above left) the arms originate directly from the nucleus, or bulge, where in the barred spirals (see right) there is a bar of material that runs through the nucleus that the arms emerge from. Both of these types are given a classification according to how tightly their arms are wound. The classifications are a, b, c, d ... with "a" having the tightest arms. In type "a", the arms are usually not well defined and form almost a circular pattern. Sometimes you will see the classification of a galaxy with two lower case letters. This means that the tightness of the spiral structure is halfway between those two letters.

3) Irregular galaxies have no regular or symmetrical structure. They are divided into two groups, Irr I and IrrII. Irr I type galaxies have HII regions, which are regions of elemental hydrogen gas, and many Population I stars, which are young hot stars. Irr II galaxies simply seem to have large amounts of dust that block most of the light from the stars. All this dust makes is almost impossible to see distinct stars in the galaxy.

Spirals:

Ellipticals:

Irregulars:

Dwarf Irregulars:

2) Irregular galaxies are 20% among the other

3)

Before people knew that galaxies were separate objects outside our Milky Way, many of the brighter or more prominent ones were cataloged in with the groups of various fuzzy things in the sky, which includes stuff like planetary nebulae, star forming regions, globular clusters and other non-galaxy things. When you look at the names of some of these objects today, you can see a galaxy listed right next to a globular cluster or a planetary nebula. Later catalogs were compiled which tried to include only galaxies or were put together by specialized telescope surveys, so some names are really screwy. In general you'll only run across galaxies that are listed in the common catalogs. There are of course galaxies that have proper or cute names like Andromeda, the Large Magellanic Cloud, the Antenna galaxies, etc., but these are pretty rare. More often you'll see a name from one of the catalogs like the following -

From this little discussion, you can see how original and creative astronomers can be with naming galaxies. Actually, with there being thousands of galaxies visible to most large telescopes, giving them all individual names like "Bob" and "Becky" would be a bit too difficult. You must remember that the catalogs mentioned above are just the more commonly used ones, and these catalogs include a lot of non-galaxy objects. The Andromeda Galaxy is so commonly surveyed and studied that it has more than 20 different catalog names or designations

5) The Big Bang model states that the earliest state of the Universe was extremely hot and dense and that it subsequently expanded. The model is based on general relativity and on simplifying assumptions such as homogeneity and isotropy of space. A version of the model with a cosmological constant (Lambda) and cold dark matter, known as the Lambda-CDM model, is the simplest model that provides a reasonably good account of various observations about the Universe. The Big Bang model accounts for observations such as the correlation of distance and redshift of galaxies, the ratio of the number of hydrogen to helium atoms, and the microwave radiation background.

The initial hot, dense state is called the Planck epoch, a brief period extending from time zero to one Planck time unit of approximately 1043 seconds. During the Planck epoch, all types of matter and all types of energy were concentrated into a dense state, where gravitation is believed to have been as strong as the other fundamental forces, and all the forces may have been unified. Since the Planck epoch, the Universe has been expanding to its present form, possibly with a very brief period of cosmic inflation which caused the Universe to reach a much larger size in less than 1032 seconds.

After the Planck epoch and inflation came the quark, hadron, and lepton epochs. Together, these epochs encompassed less than 10 seconds of time following the Big Bang. The observed abundance of the elements can be explained by combining the overall expansion of space with nuclear and atomic physics. As the Universe expands, the energy density of electromagnetic radiation decreases more quickly than does that of matter because the energy of a photon decreases with its wavelength. As the Universe expanded and cooled, elementary particles associated stably into ever larger combinations. Thus, in the early part of the matter-dominated era, stable protons and neutrons formed, which then formed atomic nuclei through nuclear reactions. This process, known as Big Bang nucleosynthesis, led to the present abundances of lighter nuclei, particularly hydrogen,deuterium, and helium. Big Bang nucleosynthesis ended about 20 minutes after the Big Bang, when the Universe had cooled enough so that nuclear fusion could no longer occur. At this stage, matter in the Universe was mainly a hot, dense plasma of negatively charged electrons, neutral neutrinos and positive nuclei. This era, called the photon epoch, lasted about 380 thousand years.

Eventually, at a time known as recombination, electrons and nuclei formed stable atoms, which are transparent to most wavelengths of radiation. With photons decoupled from matter, the Universe entered the matter-dominated era. Light from this era could now travel freely, and it can still be seen in the Universe as the cosmic microwave background(CMB). After around 100 million years, the first stars formed; these were likely very massive, luminous, and responsible for the reionization of the Universe. Having no elements heavier than lithium, these stars also produced the first heavy elements through stellar nucleosynthesis.The Universe also contains a mysterious energy called dark energy; the energy density of dark energy does not change over time. After about 9.8 billion years, the Universe had expanded sufficiently so that the density of matter was less than the density of dark energy, marking the beginning of the present dark-energy-dominated era. In this era, the expansion of the Universe is accelerating due to dark energy.

3) Irregular galaxies have no regular or symmetrical structure. They are divided into two groups, Irr I and IrrII. Irr I type galaxies have HII regions, which are regions of elemental hydrogen gas, and many Population I stars, which are young hot stars. Irr II galaxies simply seem to have large amounts of dust that block most of the light from the stars. All this dust makes is almost impossible to see distinct stars in the galaxy.

• Spiral Galaxies (~75%)
• Elliptical Galaxies (20%)
• Irregular Galaxies (5%)

Spirals:

• Range is ~10-20% gas
• On-going star formation in the disks
• Mix of Pop I and Pop II stars

Ellipticals:

• Very little or no gas or dust
• Star formation ended billions of years ago
• See only old Pop II stars

Irregulars:

• Can range up to 90% gas
• Often a great deal of on-going star formation
• Dominated by young Pop I stars

Dwarf Irregulars:

• Very metal poor (<1% solar)
• Forming stars for the first time only now.

2) Irregular galaxies are 20% among the other

3)

Before people knew that galaxies were separate objects outside our Milky Way, many of the brighter or more prominent ones were cataloged in with the groups of various fuzzy things in the sky, which includes stuff like planetary nebulae, star forming regions, globular clusters and other non-galaxy things. When you look at the names of some of these objects today, you can see a galaxy listed right next to a globular cluster or a planetary nebula. Later catalogs were compiled which tried to include only galaxies or were put together by specialized telescope surveys, so some names are really screwy. In general you'll only run across galaxies that are listed in the common catalogs. There are of course galaxies that have proper or cute names like Andromeda, the Large Magellanic Cloud, the Antenna galaxies, etc., but these are pretty rare. More often you'll see a name from one of the catalogs like the following -

• Messier catalog (M1, M2, etc.), which has listed for the Andromeda Galaxy M31. There are about 110 objects in this catalog.
• New General Catalog (NGC 1, NGC 2, etc.), which all start with NGC, and in this catalog the Andromeda Galaxy is known as NGC 224. There are nearly 8000 objects in this catalog.
• Index Catalog (IC 1, IC 2, etc.), sort of an extension of the NGC catalog, and a galaxy in this system could be called something as exotic as IC 3242. There are about 5500 objects in this catalog.

From this little discussion, you can see how original and creative astronomers can be with naming galaxies. Actually, with there being thousands of galaxies visible to most large telescopes, giving them all individual names like "Bob" and "Becky" would be a bit too difficult. You must remember that the catalogs mentioned above are just the more commonly used ones, and these catalogs include a lot of non-galaxy objects. The Andromeda Galaxy is so commonly surveyed and studied that it has more than 20 different catalog names or designations

5) The Big Bang model states that the earliest state of the Universe was extremely hot and dense and that it subsequently expanded. The model is based on general relativity and on simplifying assumptions such as homogeneity and isotropy of space. A version of the model with a cosmological constant (Lambda) and cold dark matter, known as the Lambda-CDM model, is the simplest model that provides a reasonably good account of various observations about the Universe. The Big Bang model accounts for observations such as the correlation of distance and redshift of galaxies, the ratio of the number of hydrogen to helium atoms, and the microwave radiation background.

The initial hot, dense state is called the Planck epoch, a brief period extending from time zero to one Planck time unit of approximately 1043 seconds. During the Planck epoch, all types of matter and all types of energy were concentrated into a dense state, where gravitation is believed to have been as strong as the other fundamental forces, and all the forces may have been unified. Since the Planck epoch, the Universe has been expanding to its present form, possibly with a very brief period of cosmic inflation which caused the Universe to reach a much larger size in less than 1032 seconds.

After the Planck epoch and inflation came the quark, hadron, and lepton epochs. Together, these epochs encompassed less than 10 seconds of time following the Big Bang. The observed abundance of the elements can be explained by combining the overall expansion of space with nuclear and atomic physics. As the Universe expands, the energy density of electromagnetic radiation decreases more quickly than does that of matter because the energy of a photon decreases with its wavelength. As the Universe expanded and cooled, elementary particles associated stably into ever larger combinations. Thus, in the early part of the matter-dominated era, stable protons and neutrons formed, which then formed atomic nuclei through nuclear reactions. This process, known as Big Bang nucleosynthesis, led to the present abundances of lighter nuclei, particularly hydrogen,deuterium, and helium. Big Bang nucleosynthesis ended about 20 minutes after the Big Bang, when the Universe had cooled enough so that nuclear fusion could no longer occur. At this stage, matter in the Universe was mainly a hot, dense plasma of negatively charged electrons, neutral neutrinos and positive nuclei. This era, called the photon epoch, lasted about 380 thousand years.

Eventually, at a time known as recombination, electrons and nuclei formed stable atoms, which are transparent to most wavelengths of radiation. With photons decoupled from matter, the Universe entered the matter-dominated era. Light from this era could now travel freely, and it can still be seen in the Universe as the cosmic microwave background(CMB). After around 100 million years, the first stars formed; these were likely very massive, luminous, and responsible for the reionization of the Universe. Having no elements heavier than lithium, these stars also produced the first heavy elements through stellar nucleosynthesis.The Universe also contains a mysterious energy called dark energy; the energy density of dark energy does not change over time. After about 9.8 billion years, the Universe had expanded sufficiently so that the density of matter was less than the density of dark energy, marking the beginning of the present dark-energy-dominated era. In this era, the expansion of the Universe is accelerating due to dark energy.

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