Star's fountain of Youth

Orion and its Nebula

There are few constellations which match the stardom of Orion. Especially for the Northerners, it is a constellation which is visible on almost daily basis. Below its belt, along the 'Sword of Orion', there is another famous celestial object, Orion Nebula. It is sort of a stellar nursery, where the new stars are taking birth. The Orion Nebula is a fabulous place to study stellar birth. For two reasons:

• It is near by- 1500 ly only
• It has ample diversity- It has a great mixture of low and high-mass stars.

Other than this, it contains a vast quantity of gas and dust forming something called Molecular Cloud. These clouds contain the raw material for new stars. What's happening in Orion must have happened in Solar System or in Milky Way Galaxy as a whole during the early days of Solar Birth.

We frequently post such images and facts at our Facebook page, Astro NITR and our website. Keep yourself updated with Astronomy facts and wonders.

The Glow From the Past

Almost all of our knowledge about the Universe relies heavily on the understanding of the Cosmic Microwave Background Radiation (CMBR). CMBR has now become an important source of information for the nature, evolution and constituents of the Universe. Its discovery was one of triumphs of the classical Big Bang theory, and the discovery of fluctuations in its isotropy was among the most convincing evidences for the Inflationary Big Bang model. Naturally, any discussion on cosmology doesn’t start without referring to the CMBR, for it’s the most ancient light that we can see.

Background about Background Radiation

To understand the origin of the CMBR we have to go back a little to the expansion of Universe. In 1924, Edwin Hubble discovered that the Universe as a whole is expanding. Based on these observations he established a law, Hubble’s law, according to which the farther a galaxy is, the faster is it receding from us. So, as one rewinds the clock, all the matter (or energy) that we see in the Universe, must have been localized at a tiny place. It is the extrapolation of the Hubble’s law back in time, which gives the age of the Universe [i.e. from the time of the Big Bang] to be 13.75 billion years.

The farther we look into the space, we peer further back in time. The light from the distant galaxies are hugely red-shifted. Computer simulations conclude that the stars with the highest redshifts, emerged when the universe was about 100 million years old (Turner, 2009). Before that time, the Universe went through a time where there was no light. There were no stars, no galaxies - just a featureless gruel of hydrogen, helium and dark matter. Fluctuations in the mass densities, caused initial clumping of matter, which became larger and larger, finally culminating into stars and galaxies that we see (This description is oversimplified. Formation of the early Universe is in itself an interesting topic). Radiation from these stars and galaxies are greatly red-shifted and are detectable only in radio frequency range.

After-glow of a Hot Past

Beyond these dark ages is the glow of the hot Big Bang. This has a redshift of about 1,100. That redshift pushes the frequency of this radiation, right into the Microwave region. What one sees, is a wall of microwave radiation which fills the sky almost uniformly. And this is the Cosmic Microwave Background Radiation (CMBR). CMBR was discovered in 1964 by Arno Penzias and Robert Wilson. Using Hubble’s Law, one concludes that this time corresponds to about 380,000 years after the Big Bang.

CMBR thus gives us a glimpse of how the Universe looked in the early epochs, and the physical processes which were predominant during that time. Any model of the early Universe should explain the features of the CMBR, and any predictions from the model should agree with CMBR observations. It thus gives us the way to model the Universe and understand its evolution.

Features of CMBR

The intensity of CMBR is same in all direction. Penzias and Wilson measured the wavelength of this radiation to be around 7.3 cm. If one assumes that the radiation spectrum is from a blackbody, this corresponds to a temperature of 3.5 ± 1.0 K. These predictions have been refined and the currently accepted temperature is 2.73 K.

There are broadly three different features of CMBR which provide direct and indirect evidence about the origin of the Universe

Angular Dependence– Intensity of radiation is almost isotropic, but has a small dependence on the direction.

Spectrum– The radiation is almost Planckian [i.e. obeys Planck’s law of radiation], but has slight deviation.

Polarization– The degree of polarization is slightly different in different directions.

In next post, we shall discuss about the origin of CMBR, and each of its features, especially the isotropy, in detail.

Citations

Turner, M. S. (2009, September). The Origin of the Universe. Scientific American.

 

 

Why Hubble is famous?

 

The topic of Expansion of the Universe doesn’t start without mentioning the name of the American Scientist Edwin Powell Hubble. He is considered as the father of observational cosmology for his significant contributions that revolutionized our understanding of the Universe. While everyone knows that he discovered that the Universe is expanding, and provided the evidence for Lemaitre’s model of ‘Primordial Soup’ [nascent version of the modern Big Bang Theory], there are peels of factoids which remain un-discussed. To start with, Hubble was not the first person to find observational evidence for the expansion of the Universe.

In 1920s, Vesto Slipher, another American astronomer, used spectra of stars to measure the velocities of nearby galaxies [during that time, galaxies were not well-defined]. His method of observation was based on Doppler Effect. Waves of light from a star moving toward Earth are compressed, shortening the wavelength and making the light bluer. Light waves from an object moving away from us are stretched, making the wavelength longer and the light redder. By measuring the lengthening or compression of the light waves from distant galaxies, Slipher was able to determine whether they were moving toward us or away from us and at what speed (Krauss & Scherrer, 2008). Slipher concluded that almost all the galaxies were moving away from us.

But Hubble, not Slipher, is credited with the discovery of expanding Universe. Well, this isn’t a scientific conspiracy, because Hubble did two things which justify his fame. Hubble determined the distances of the galaxies along with the velocities. And this extra parameter led to important implications:

1. First Hubble showed that galaxies were so far away that they really were independent collections of stars, just like our own galaxy
2. Second, he discovered that the velocity of recession of a galaxy is directly proportional to the distance of the galaxy. [The proportionality constant, Hubble Constant, gives the most reliable measurement for the age of the Universe.]

So, not only did he change the notion that our galaxy is the ‘island universe’ surrounded by eternity of void, he also gave results which match the theoretical expansion rates. His discoveries strengthen the view that in past there must have been something which caused the expansion to begin with, as proposed by Big Bang theory. Hubble also calculated this epoch of expansion to be around 2 billion years [current estimates are 13.75 billion years].

He had won many awards, including the Gold medal from Royal Astronomical Society (Ravindranath, 2009). Many would argue that he would have been a strong contender for the Nobel Prize, if Astronomy was considered a part of Physics, rather than a field of its own. In 1990, the Hubble Space Telescope (HST) was launched. HST is among the most powerful optical telescopes (more on that later).

 

Citations

Krauss, L. M., & Scherrer, R. J. (2008, March). The End of Cosmology. Scientific American.

Ravindranath, S. (2009, March). Edwin Powell Hubble. Resonance.