List three pieces of evidence that support the existence of interstellar matter

Question

List three pieces of evidence that support the existence of interstellar matter (not dark matter) even in regions where we do not observe nebulae.cDescribe why energy must flow in stars and describe two ways in which it can flow in normal stars. List three reasons why pulsars are not always associated with supernova remnants. List the most compelling evidence (in your opinion) for the big bang. Provide a brief argument to support your choice of evidence as the most compelling. Name three spiral tracers and explain why they have to be short lived. Provide a brief summary of the argument used to first convincingly show that the solar system is not in the central region of the Galaxy.

Explanation / Answer

1.List three pieces of evidence that support the existence of interstellar matter (not dark matter) even in regions where we do not observe nebulae

Ans.

A nebula is a glowing gas-cloud or a dust-cloud reflecting the light of nearby stars.Interstellar matter acts as an intermediate matter between stellar and galactic scales.The spectra of many stars display absorption lines. These absorption lines are very narrow, suggestingthat they originate from a colder and less dense medium between the star and astronomers' telescope.When a light beam passes through an interstellar medium made up of mostly gas, the atoms in the gasabsorb appropriate photons from the beam. This leaves narrow dark absorption features in the spectraof the stars.Extinction is the dimming of starlight between the source and the observer through absorption andscattering by interstellar matter. Dust both absorbs and scatters light, thus causing the source of light toappear both dimmer and more spread out than it actually is. The phenomenon of extinction is evidencefor the presence of interstellar matter.Furthermore, interstellar matter attenuates blue light more easily than red light. If a high amount ofinterstellar matter separates the star from the observer, the star will appear redder than it actually is.This phenomenon is called interstellar reddening. The phenomenon of interstellar reddening is evidencefor the presence of interstellar matter.

2.Describe why energy must flow in stars and describe two ways in which it can flow in normal stars.

Ans. Energy flows in stars in order to balance the force of gravity and maintain stability. If energy did not flow , the star would collapse in on itself due to gravitational forces.

3.List three reasons why pulsars are not always associated with supernova remnants.

Ans.Most pulsars are created by Type II supernovae, but out of all the pulsars detected yet (around two thousand), only a few are associated with supernova remnants. There are three main reasons for this. Firstly, some pulsars move at very high velocities through space and leave their supernovae remnants behind rather quickly (in several tens of thousands of years). Those pulsars have high velocities because they were given a strong kick at birth. This strong kick may be violent turbulence in the core of a massive star during an assymetrical explosion or the flinging of two massive stars apart during a supernova explosion in a binary system. Secondly, the beams of some pulsars within supernova remnants never sweep over Earth and are not detected by astronomers. Thus, from Earth, the supernova remnant does not seem to be associated with a pulsar. Thirdly, pulsars live for more time than their supernova remnants. Pulsar remain detectable for around 10 million years, while their supernova remnants dissipate into the interstellar medium after around 50,000 years. It is not surprising that pulsars over 50,000 years old have already lost their supernova remnants.

4.List the most compelling evidence (in your opinion) for the big bang.Provide a brief argument to support your choice of evidence as the most compelling.

Ans.The most compelling observational evidence for the Big Bang is the expansion of the universe as shown by the Hubble Diagram. The Big Bang is defined as the initial moment of expansion of the universe. This expansion continued after the “moment” of the Big Bang and is still going on. Galaxies and quasars in all directions are receding from astronomers' observation point, i.e are redshifting. By plotting the redshift of different galaxies on the Hubble Diagram, it has been found that the farther a galaxy is from us, the faster it is receding from us (i.e its redshift is higher). A lineal relation exists between the velocity of redshift and distance. The galaxies themselves are not moving, but the space between them is expanding. The light from the light-source and the observer gets stretched into longer wavelengths as it travels through the expanding space. As time goes, the space separating the galaxies becomes larger and larger. The second most compelling evidence of the Big Bang, the uniform presence of cosmic microwave background radiation (CMBR), is dependent on the expansion of the universe. The original blackbody radiation from the Big Bang has expanded with the universe to uniformly fill it. The wavelength of that radiation has also been redshifted with the expansion; while it originally had a temperature of 3000 K, it now appears to have a temperature of 2.7 K.

5.Name three spiral tracers and explain why they have to be short lived.

Ans.All spiral tracers share the characteristics of being young and bright. Three spiral tracers are clouds of ionized hydrogen (emission nebulae), O and B stars, and open clusters. New hot stars are formed in the spiral arms, and those stars heat and ionize clouds of hydrogen gas into emission nebulae. Areas of ionized hydrogen gas in the spiral arms appear bright red (because the predominant emission line of hydrogen here is red). The youngest stars formed in those spiral arms are O and B stars. O and B stars are big, hot and bright blue. They give the spiral arms of a galaxy a bright blue color. Open clusters of stars are relatively young (compared to globular clusters) and are confined to the disk of a galaxy. The youngest open clusters are closest to the spiral arms. Young open clusters contain young (often O and B) stars and are bright. The brightness of ionized hydrogen clouds, O and B stars, and open clusters together help map spiral arms.

Emission nebulae are heated and ionized by O and B stars. The youngest bright stars in young open clusters are O and B stars. Thus, those two first spiral tracers are dependent on O and B stars. If O and B stars are short-lived, the role of emission nebulae and young open clusters as spiral tracers are short-lived too. O and B star are indeed short-lived. O stars live for only a few million years, while B stars can live up to only about 25 million years. O stars have masses of around 20-100 solar masses and B stars have masses of around 3-18 solar masses. Their tremendously high masses makes those stars burn all of their nuclear fuels very quickly.

6.Provide a brief summary of the argument used to first convincingly show that the solar system is not in the central region of the Galaxy.

Ans.Early in the 20th century, the American astronomer Harlow Shapley mapped the distances and direction of Cepheid variable stars in globular clusters. Prior to mapping globular clusters, Shapley had noticed that those are concentrated in the direction of the constellation Sagittarius and Scorpius on the celestial sphere. He had guessed (assumed) that this concentration of globular clusters are controlled by the combined gravitational field of the entire larger star system. He realized that if his assumption was correct, he could know about the size and extent of the entire star system by studying globular clusters. He studied the distances and distribution to globular clusters using H.S Leavitt's period-luminosity law and a statistical method he had devised to replace the apparent magnitude on the period-luminosity diagram by absolute magnitudes. He searched catalogs for the proper motions of some Cepheid variable stars, and used his statistical method to find their average distance and average absolute magnitude from those proper motions. Then, by comparing the stars' absolute magnitudes with their apparent magnitudes using the formula mv - Mvb = 5 + log10 (d), he was able to find the distances to the globular clusters they were in. He plotted the distances and directions of those globular clusters and discovered that their distribution are not centered around the solar system. The concentrated center of the globular clusters was indeed situated near Sagittarius and Scorpius, many thousands of light-years away from the solar system. The solar system is only in the 'suburbs' of the galactic star system, about two-thirds of the way out from the center.

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