by Madhurani Chavan | फेब्रुवारी 25, 2022 | English, खगोलशास्त्र, मधुरानी
A nova and supernova are both bright events in the sky that are generally visible during the night to the naked eye. Until very recently, they were used very interchangeably. However, recent discoveries have found that the two have completely different causes.
Origin
Tyco Brahe coined the term “nova” after observing the supernova SN 1572 in the constellation Cassiopeia in the 16th century. He described it in his book De Stella nova, which is Latin for “concerning the new star”. It was from here that the term nova was coined, though strictly speaking, the observed event was a supernova.
Cataclysmic nuclear explosion
Innova is a cataclysmic nuclear explosion on a white dwarf. A nova is caused by the accretion of hydrogen onto the surface of the star. Due to the closeness, hydrogen accumulates on the surface of a white dwarf in a binary system, after being bled off from the larger star. It then ignites and starts nuclear fusion in a raging manner.
Mechanism
The gases are compacted on the white dwarf’s surface by its intense gravity, compressed and heated to very high temperatures as additional material is drawn in. As the white dwarf is dying and composed of degenerate matter, it does not absorb the material and inflate – as another star like the Sun would.
For most binary system parameters, the hydrogen-burning is thermally unstable once it has reached the required fusion temperatures. As a result, it rapidly converts a large amount of hydrogen into other heavier elements in a runaway reaction. This liberates an enormous amount of energy, blowing the remaining gases away from the white dwarf’s surface and producing an extremely bright outburst of light.
- compiled by Madhurani Chavan
by Madhurani Chavan | फेब्रुवारी 25, 2022 | English, खगोलशास्त्र, मधुरानी
Supernova is a stellar explosion that briefly outshines an entire galaxy. The amount of energy that is radiated is as much as the Sun or any ordinary star is expected to emit over its entire lifespan but over a brief burst. The supernova fades from view over a period of weeks or months.
Earliest observation and discovery
The earliest recorded supernova was SN 185, which was viewed by Chinese astronomers in 185 AD. SN 1006 is the brightest recorded supernova in human history. Earlier, supernovae were considered as a brighter form of novae. Walter Baade and Fritz Zwicky at Mount Wilson Observatory did early work on what was originally believed to be simply a new category of novae. The term “super-novae” was first used during 1931 lectures held at Caltech by this pair. The hyphen had been lost and the modern name was in use by 1938.
Rare events
Supernova is a relatively rare event within a galaxy, occurring about thrice in a century in the Milky Way. Supernova cannot be predicted with any meaningful accuracy and must be observed in progress. Thus, both amateurs and professionals conduct extensive supernova searching. Supernova searches fall mainly into two classes: those searches focussed on relatively nearby events and those looking for explosions farther away.
Format
Supernova discoveries are reported to the IAU’s Central Bureau for Astronomical Telegrams. The name of a supernova is in the following format: SN followed by the year of discovery, suffixed with a one or two-letter designation.
Compiled by: Madhurani Chavan
by Madhurani Chavan | फेब्रुवारी 25, 2022 | English, खगोलशास्त्र, मधुरानी
Most stars consist of hydrogen as their primary component. However, there are certain stars that are extremely low or almost devoid of hydrogen. This category of stars is termed “hydrogen-deficient stars”.
Hydrogen deficient stars
Cool carbon stars like R Coronae Borealis, helium-rich spectral type WC and transition stars like PG 1159 are all hydrogen deficient. An extreme helium star, often abbreviated as “EH”, is a low-mass supergiant that is almost devoid of hydrogen.
Discovery
Daniel M. Popper discovered the first known extreme helium star at the McDonald Observatory in Austin, USA, in 1942. By 1996, 25 possible helium stars were identified. This was further narrowed to 21 by 2006. Extreme helium stars are characterized as those that display no lines of hydrogen in their spectrum, but strong helium lines as well as the presence of carbon and oxygen.
Size and composition
The known extreme helium stars are “supergiants”. Hydrogen is less abundant by a factor of 10,000 or more, and surface temperatures range from 9,000 to 35,000 K. There are two popular theories as to how these stars are formed and why they have their unique composition.
The double-degenerate (DD) model :
This explains stars forming in a binary system. It has a small helium white dwarf and a more massive carbon-oxygen white dwarf. Gravity causes them to collide and form a dwarf that ignites into a supergiant.
The final-flash (FF) model :
It says that helium ignites in a shell around the core, causing the dwarf to rapidly expand.
by Madhurani Chavan | फेब्रुवारी 25, 2022 | English, खगोलशास्त्र, मधुरानी
Red giant is a luminous giant star of low or intermediate mass. Its mass usually ranges between 0.3 to eight times that of our Sun. These stars are usually in a very late phase of stellar evolution. Red giants have radii tens to hundreds of times larger than that of the Sun. Their outer envelope is lower in temperature, about 5,000 K and below.
Formation
When a star initially forms from a collapsing molecular cloud in the interstellar medium, it primarily contains hydrogen and helium, with trace amounts of “metals”, i.e., any element heavier than helium. When the star exhausts the hydrogen fuel in its core, nuclear reactions can no longer continue, and thus the core begins contracting due to its own gravity. This causes the remaining hydrogen to undergo fusion in a shell around the core at a faster rate. The outer layers of the star then expand greatly. This begins the red giant phase of a star’s life.
Colour and naming
Since the expansion of the star greatly increases its surface area, red giants tend to be cooler and burn with an orange hue. Despite their name, they are closer to orange in reality. The M-type stars HD 208527, HD 220074 and K-giants including Pollux, Gamma Cephei and lota Draconis are some examples of red giants with planets.
Life around red giants
It has traditionally been suggested that life could not evolve on planets orbiting them. However, current research suggests that there would be a habitable zone at twice the distance from Earth to Sun for a billion years. At a distance of nine AU, such a habitable zone would only exist for 100 million years. As of June 2014, 50 giant planets have been discovered around the giant stars. These giants are much larger than those found around sun-sized stars.
- compiled by Madhurani Chavan
by Madhurani Chavan | फेब्रुवारी 25, 2022 | English, खगोलशास्त्र, मधुरानी
VY Canis Majoris or VY CMa is a red hypergiant star. It is located in the constellation Canis Major. It is one of the largest and brightest red hypergiants observed so far. It has a diameter of 1800 solar radii. This star emits energy very quickly and therefore, only exists for a few million years. It is estimated to be 4900 light-years away from Earth. This star shows periodic light changes that last for approximately 2200 days.
The crimson star
The first known recorded observation of VY Canis Majoris is in the star catalog of Jérôme Lalande, who recorded it on 7th March 1801. Since 1847, VY CMa has been known to be a crimson star. Originally, University of Minnesota Professor Roberta M. Humphreys approximated that the radius of VY CMa is 1800-2100 times that of the Sun. This would make it the largest known star based on its radius.
A big star
There have been conflicting opinions of the properties of VY CMa. A commonly held theory states that the star is a very large and luminous red hypergiant. However, various larger estimates of the size and luminosity fall outside the bounds of current stellar theory. In another theory, the star is a normal red supergiant with a radius around 600 times that of our Sun.
The surface of VY CMa
This star also illustrates the conceptual problem of defining the “surface” of very large stars. This is very important for multiple reasons, including determining its radius and thus its size. It is a hundred thousand times less dense than the atmosphere of Earth (air) at sea level. Its average density is 0.000005 to 0.000010 kg per metre³. Additionally, the star is constantly losing mass at an astounding rate. The boundary of such a star is usually defined by its “Roseland Radius”, which is based on its opaqueness to light.
Compiled by: Madhurani Chavan
by Madhurani Chavan | फेब्रुवारी 25, 2022 | खगोलशास्त्र, मधुरानी
Sirius is the brightest star system in Earth’s night sky. The name “Seirios”, means “glowing” or “scorcher”. What appears to be a single star to the naked eye is actually a binary system of the white dwarf. This system is formally known as Alpha Canis Majoris or alpha CMa.
Sirius in ancient Egypt
Sirius is recorded in the earliest astronomical records. It was known in ancient Egypt as “Sopdet”, which is written in Greek as “Sothis”. The Egyptians based their calendar on the heliacal rising of Sirius. This heliacal rising marked the flooding of the Nile in ancient Egypt and the “dog days” of summer for the ancient Greeks.
Sirius to Polynesians
To the Polynesians in the southern hemisphere, it marked winter. The heliacal rising of Sirius refers to the day it becomes visible just before sunrise after moving far away from the glare of the Sun.
Dogstar
Due to its prominent position in Canis Majoris (greater Dog) constellation, Sirius is often also referred to as the “Dogstar”.
Brightness
Sirius is the brightest star system in the night sky with almost twice the brightness of the second brightest star, Canopus. However, it is not as bright as the moon, Venus, or Jupiter. In fact, even Mars and Mercury appear brighter than Sirius at times.
Types of Sirius
The two stars of the Sirius system are termed Sirius A is a white main-sequence star with an estimated surface temperature of 9,940 K. It has an estimated mass that is twice that of the Sun. Sirius B is a star that has already evolved off the main sequence and transformed into a white dwarf. It has a mass almost equal to that of the Sun. It is one of the most massive white dwarfs known to us.
Red, blue, or white?
Greek astronomer Ptolemy had described the star of Sirius to be red in color around 1 CE and ancient China has been describing it as white since 4 CE. Astronomers believe that this star keeps changing its color. However, the reason is still being debated.
Compiled by: Madhurani Chavan
