Stars and Their Types |Classifications|Characteristics|Cluster|Novae-Supernovae

STARS 

Stars are huge celestial bodies made mostly of hydrogen and helium that produce light and heat from the churning nuclear forges inside their cores. Aside from our sun, the dots of light we see in the sky are all light-years from Earth. Most of the stars in our galaxy, including the sun, are categorized as Main-Sequence Stars. They exist in a stable state of nuclear fusion, converting hydrogen to helium and radiating x-rays. This process emits an enormous amount of energy, keeping the star hot and shining brightly.

Main-Sequence Stars: 

Main sequence stars fuse hydrogen atoms to form helium atoms in their cores. These stars can range from about a tenth of the mass of the sun to up to 200 times as massive. Stars start their lives as clouds of dust and gas.

CHARACTERISTICS OF STARS

Brightness

The brightness of a star depends on its size, temperature, and distance from the observer.

Astronomers describe star brightness in terms of magnitude and luminosity.

Luminosity is the amount of light that a star radiates. The size of the star and its surface temperature determine its luminosity.

The Apparent magnitude of a star is its perceived brightness, factoring in size and distance, while Absolute magnitude is its true brightness irrespective of its distance from earth.

Color

Stars come in a range of colors, from reddish to yellowish to blue. The color of a star depends on surface temperature.

A star might appear to have a single color, but actually emits a broad spectrum of colors, potentially including everything from radio waves and infrared rays to ultraviolet beams and gamma rays. Different elements or compounds absorb and emit different colors or wavelengths of light, and by studying a star's spectrum, one can divine what its composition might be.

Distance 

Distances on Earth’s surface are often measured in kilometers. However, distances to the stars are so large that kilometers are not very practical units. Astronomers use a unit called the light-year to measure distances between the stars. A light-year is a distance that light travels in one year, about 9.5 million million kilometers.

Standing on Earth looking up at the sky, it may seem as if there is no way to tell how far away the stars are. However, astronomers have found ways to measure those distances. Astronomers often use parallax to measure distances to nearby stars. Parallax is the apparent change in position of an object when you look at it from different places.

Surface Temperature 

Astronomers measure a star's temperature on the Kelvin scale. Zero degrees on the Kelvin scale is theoretically absolute and is equal to -273.15 degrees Celsius. The coolest, reddest stars are approximately 2,500 K, while the hottest stars can reach 50,000 K. Our sun is about 5,500 K.

Size 

Astronomers measure the size of a given star in terms of our own sun's radius. Thus, a star that measures 1 solar radii would be the same size as our sun. The star Rigel, which is much larger than our sun, measures 78 solar radii. A star's size, along with its surface temperature, will determine its luminosity. 

Mass 

A star's mass is also measured in terms of our own sun, with 1 equal to the size of our sun. For instance, Rigel, which is much larger than our sun, has a mass of 3.5 solar masses. Two stars of a similar size may not necessarily have the same mass, as stars can vary greatly in density.

STAR CLUSTERS

Star clusters are a group of stars that gravitationally bound together for some length of time star cluster is much closer than the average distance between stars and the rest of the galaxy. basically, here are two types of star clusters

1.Globular clusters

2.Galactic or Open clusters.

Globular clusters 

Globular clusters contain several thousand to one million stars in spherical, gravitationally-bound systems. Located mostly in the halo surrounding the galactic plane they comprise the oldest stars in the galaxy. These Population II stars are highly evolved but with low metallicities. Clusters are so old that any star higher than a G or F-class will have already evolved off the main sequence. There is little free dust or gas found in globular clusters so no new star formation is taking place in them. Stellar densities within the inner regions of a globular cluster are very high compared with regions such as those around the Sun. 

Galactic or Open clusters

Open clusters are so-named due to the fact that the individual component stars are easily resolved through a telescope. Some examples such as the Hyades and Pleiades are so close that the individual stars can be clearly distinguished by the naked eye. They are sometimes called galactic clusters due to their location on the dusty spiral arms on the plane of spiral galaxies. Stars in an open cluster have a common origin - they formed from the same initial giant molecular cloud. Clusters typically contain a few hundred stars though this can vary from as low as a few dozen up to a few thousand.

TYPES OF STARS

Protostars 
T Tauri stars 

Dwarfs
Giants 

PROTOSTARS

Long before stars begin fusing hydrogen at their centers when they are still just hot overdensities in the process of contracting we call them Protostars. Space is not empty after all its actually full of gas and dust which is called the interstellar medium the gas is mostly made up of atoms of hydrogen though small amounts of heavier elements can be found floating through space as well in some regions the interstellar medium is assembled into a big cloud of dust and gas called a nebula stars begin very small simple particles and vast clouds of dust and gas. these nebulae remain cold and monotonous for ages until everything stirs up when a newcomer speeds through as particles collide they begin to form clumps eventually a clump reaches more mass and therefore a stronger gravitational pull attracting even more particles from surrounding cloud.

hence the center grows denser and hotter and over millions of years the clump grows into a small dense body called a protostar. A protostar looks like a star but its core is not yet hot enough for fusion to take place the brightness. Protostars are very difficult to be seen in the visible spectrum because they are surrounded by dust and gas which blocks the light they emit this stage of stellar evolution may last for between 1,00,000 and 10 million years depending on the size of the star being formed. slowly when the stellar winds and radiation blow away the gas and dust when the surrounding shell is all cleared the star will enter the T Tauri phase.

T TAURI STARS 

T Tauri stars are less than 10 million years old a star begins its life as a protostar still enveloped in its molecular cloud over the years the newborn star accretes new material from the surrounding area and develops a protoplanetary disc slowly stiller winds and radiation blow away the gas and dust. when the surrounding shell is all cleared up the star enters the T Tauri phase. because T Tauri stars are so young. these stars are among the first young ones to be identified because the earlier stages still embedded in their birth clouds were blocked from optical observations by the dust. So, they now become visible at optical wavelengths.

T Tauri stars are variable in nature they show both periodic and random fluctuations in their brightnesses. these types of stars can be quite active rotating at a period of a few days in comparison to the 30 days it takes for the sun to complete one turn many also spit out intense and powerful stellar winds are thought to be powered by material falling onto the central star via the accretion disk or protoplanetary disk to surround many of them. planets will also form from this protoplanetary disk and some may survive to form a planetary system surrounding the newborn star.

DWARF STARS

A dwarf star is a star of relatively small size and low luminosity. Most main sequence stars are dwarf stars.

Types of Dwarf Stars:

Red Dwarfs

They are relatively small in size compared to other stars and therefore they burn at a lower temperature. which helps them have a long sustainable life. Nearly 70% of the stars in-universe are red dwarf stars. but they do not shine so bright, it gets difficult for humans to see them with naked eyes. 

Yellow Dwarfs

These are medium-sized stars and therefore burn at a medium temperature. these stars appear yellow when viewed from the earth's surface. but when you look at them in the space, you will realize that they are more white than yellow. our sun is one of them. these stars become quite large just before they completely start running out of fuel.

Orange Dwarfs

These smaller and cooler dwarf stars are similar to their yellow counterparts, but they live longer than yellow dwarfs. some of these examples of orange dwarf stars are Alpha Centauri B and Epsilon Eridani.

Brown Dwarfs

Brown dwarfs are substellar objects that never accumulated enough material to become stars. They are too small to generate the heat required for hydrogen fusion. brown dwarfs constitute the midpoint between the smallest red dwarf stars and massive planets like Jupiter. they are the same size as Jupiter, but to qualify as a brown dwarf, they must be at least 13 times heavier. 

White Dwarfs

White dwarf stars less than 10 solar masses will shed their outer layers to form planetary nebulae. They will typically leave behind an Earth-sized core of less than 1.4 solar masses. this core will be so dense that the electrons within its volume will be prevented from occupying any smaller region of space (becoming degenerate). This Physical law prevents the stellar remnant from collapsing any further. the remnant is called a White Dwarf.

Black Dwarfs

once a star becomes a white dwarf, it will slowly cool to become a black dwarf. as the universe is not old enough for a white dwarf to have cooled sufficiently, no black dwarfs are thought to exist at this time.

GIANT STARS 

A giant star is a star with a substantially larger radius and luminosity than a main-sequence (or dwarf) star of the same surface temperature. 

Blue Giants

Thes are big stars and hence burn at a high temperature. These kinds of stars have a slight blueish coloration, and only about 0.7% of stars fall into this category. the largest and hottest of these stars burn through the hydrogen in their cores very quickly, causing their outer layers to expand, and their luminosity to increase. their high temperature means they remain blue for much of this expansion, but eventually they may cool to become a red giant, supergiant, or hypergiant.

Red Giant

These stars are in a late evolutionary phase. red giants would previously have been main sequence stars, with between 0.3 and 10 solar masses. smaller stars become red giants. It is because of convective heat transport, their cores cannot become dense enough to generate the heat needed for expansion. Larger stars become red supergiants or hypergiants. 

Planetary Nebula 

A planetary nebula is created when a star blows off its outer layers after it has run out of fuel to burn. These outer layers of gas expand into space, forming a nebula which is often the shape of a ring or bubble.

NEUTRON STARS

A neutron star is a very small, super-dense star. which is composed mostly of tightly-packed neutrons. It has a thin atmosphere of hydrogen. It has a diameter of about 5-10 miles (5-16 km) and a density of roughly 10 15 gm/cm3.

PULSAR

A pulsar is a rapidly spinning neutron star that emits energy in pulses.

NOVAE

The word novae means "new star" Ancient civilizations interpreted these events as the creation of a new star. Novae (plural of nova) are actually explosions that occur on stars near the end of their lives.

In a binary system, a system in which two stars closely orbit each other, one star may influence the other. If the binary system consists of a white dwarf and a main-sequence star, gas may be pulled from the main-sequence star to the white dwarf. This is like dumping lighter fluid onto hot coals; the surface of the white dwarf explodes. Depending on how energetic the explosion is, the novae will fade away in a few days to a few weeks.

SUPERNOVAE  

A supernova is the biggest explosion that humans have ever seen. Each blast is the extremely bright, super-powerful explosion of a star.

Supernovae may be divided into two broad classes, Type I and Type II, according to the way in which they detonate. Type I supernovae may be up to three times brighter than Type II; they also differ from Type II supernovae in that their spectra contain no hydrogen lines and they expand about twice as rapidly.

Type I:

The first type of supernova can happen in systems where two stars orbit one another and at least one of those stars is an Earth-sized white dwarf. A white dwarf is what's left after a star the size of our sun has run out of fuel. If one white dwarf collides with another or pulls too much matter from its nearby star, the white dwarf can explode.

Type II

Another type of supernova is caused by the “last hurrah” of a dying massive star. This happens when a star at least five times the mass of our sun goes out with a fantastic bang!

Massive stars burn huge amounts of nuclear fuel at their cores or centers. This produces tons of energy, so the center gets very hot. Heat generates pressure, and the pressure created by a star’s nuclear burning also keeps that star from collapsing.

A star is in the balance between two opposite forces. The star’s gravity tries to squeeze the star into the smallest, tightest ball possible. But the nuclear fuel burning in the star’s core creates strong outward pressure. This outward push resists the inward squeeze of gravity.

When a massive star runs out of fuel, it cools off. This causes the pressure to drop. Gravity wins out, and the star suddenly collapses. Imagine something one million times the mass of Earth collapsing in 15 seconds! The collapse happens so quickly that it creates enormous shock waves that cause the outer part of the star to explode!

Usually, a very dense core is left behind, along with an expanding cloud of hot gas called a nebula. A supernova of a star more than about 10 times the size of our sun may leave behind the densest objects in the universe—black holes. 

CLASSIFICATION OF STARS

Stars are classified by their spectra (the elements that they absorb) and their temperature. There are seven main types of stars. In order of decreasing temperature, O, B, A, F, G, K, and M.

Basically, People think star shape is a five-point shape✶✶. But actually, Stars are a Spherical shape. it's more hot and big. Stars are too away from us. So, It looks like a five-point shape.

NOTE:

All of the above details are taken from Astronomy books and videos.

By,

DEEPA S.