We have all heard about the stars and witnessed there beauty in the night sky since our childhood.
But Have you ever heard about Stellar Classification of Stars? If your answer is no, then you have
landed at the right place because this video will touch the bottom of the topic. Before
going too deep first, let us understand what exactly we mean by Stellar Classification
of Stars! In astronomy, Stellar Classification is the classification of stars based on their
spectral characteristics i.e. relation between wavelength and some other variables. Stellar
classification, scheme for assigning stars to types according to their temperatures as
estimated from their spectra.
For most of its active life, a star shines due to thermonuclear fusion of hydrogen into
helium in its core, releasing energy that traverses the star's interior and then radiates
into outer space . A star's life begins with the gravitational collapse of a gaseous nebula of
material composed primarily of hydrogen, along with helium and trace amounts of heavier elements.
When the stellar core is sufficiently dense, hydrogen becomes steadily converted into helium
through nuclear fusion, releasing energy in the process. According to the latest data,
the observable universe contains 1×1024 stars. The nearest one to our planet is our
Sun. Scientists have studied stars for quiet a time now and divided them on them on the
basis of their age and temperature. We will discuss one such classification today, which
is the stellar 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. We all see the stars twinkling but
have you ever wondered how do stars twinkle? To answer this, let us go back to our school
days where we were taught refraction of lights. Twinkling of stars is caused by the passing
of light through different layers of a tempestuous atmosphere. But in real the stars don't twinkle,
they only appear as such because of the interaction between our eyes, stars and Earth's atmosphere.
The formation of a star is not a quick process, it takes around a million of years to form
a star from an accumulation of gas and dust which collapses due to gravity in its nebulae.
Our universe has nebulas, in which new stars are being formed all the time. Though we haven’t
observed much of these stars present in our infinite universe but those which are observed
give a very good idea of their composition and properties.
Now let us go back to the base of the topic Stellar Classification. It is a scheme assigning
stars their type on the basis of their temperatures as calculated from their spectra. The classification
of temperature is done on the basis of Wien's Law regarding black body radiation. The universally
accepted system of stellar classification is a combination of two schemes, the Harvard
system, which is based on the star's surface temperature, and the MK i.e. Morgan Keenan
system, which is based on the stars luminosity. The MK also called Yerkes system, is the work
of the American astronomers W.W. Morgan, P.C. Keenan, and few others. It is based on two
sets of parameters-a refined version of the Harvard O-M scale, and a luminosity scale
of grades I-for super-giants, II-bright giants, III-normal giants, IV-sub-giants, and V-main
sequence, or dwarf, stars; further specifications may be used, such as a grade Ia for bright
super-giants and grades VI and VII for sub-dwarfs and white dwarfs, respectively. In the Harvard
system the stars are classified on as the hottest to coolest using letters such as O,
B, A, F, G, K, M where O is the hottest(25,000 K) and blue in colour and M is the coolest
(3,500 K) star which is red in colour. The letters are further subdivided from hot to
cool. An adapted and famous mnemonic for memorising this sequence of stellar classification is
“Oh Be A Fine Girl/Guy, Kiss Me”. The hotter stars are sometimes referred to as
early and the cooler as late. With the discovery of brown dwarfs, objects that form like stars
but do not shine through thermonuclear fusion, the system of stellar classification has been
expanded to include spectral types L, T, and Y.
Now let's us know about the classification in details.
Starting with the letter O: O type stars are the hottest, luminous and rarest of all main
stars. They have very complicated surrounding which makes the measurement of their spectra
difficult. Most of the massive stars lie within this particular type .Because of their massive
and giant size they have very hot cores and burn through their hydrogen fuel very quickly,
so they tend to burn brighter and are the first stars to leave the main sequence. Their
spectra have dominant lines of absorption and sometimes emission for He II lines, neutral
helium lines and prominent ionised. Further subdivision includes O3, O7, O9, etc. Spectral
type O7 is the point at which the two intensities are equal and type O3 is, the point at which
line vanishes.
Now comes, the letter B: B type stars are also very luminous and blue in colour. Their
spectra have neutral helium lines and are very energetic. Their life span is relatively
short around 40 million years. Many stars that we can see from our naked eyes are of
this type only. B-type stars are originated from an OB Association that is related to
giant molecular clouds. Now, the question here is what is an OB Association? An OB association
is a loose grouping of several thousand stars, a small fraction of which are of spectral
type O and B. B-type stars also produce 'be' type spectrum and is also detected in O and
A shell star. Be stars are a heterogeneous set of stars with B spectral types and emission
lines. According to the research it was found that they can have disk that create and can
create emission lines. They are considered to be main sequence stars, but a number of
sub-giants and giant stars are also included.
Next comes letter A: A type stars are young around a few hundred million years. They have
masses ranging from 1.4 to 2.1 times the mass of the Sun and surface temperatures between
7112 K to 11500 K. They appear bluish-white in colour, very bright and can be seen by
our naked eyes. A-type stars don't have a convective zone or convective region i.e.
a region where a layer of star is unstable to convection and as a result they lack strong
stellar winds and are not able to bear magnetic dynamo. Research proved that gigantic planets
formed around them are hard to detect using a certain method called Doppler Spectroscopy.
It is an indirect method for finding ex-o-planet i.e. planet outside of the solar system and
brown dwarfs from their radial velocity measurements.
Next is type F: F type stars are moderately hot and yellowish-white in colour, usually
weighing between 1.0 and 1.4 times the mass of the Sun. Their temperature lies between
6,000K to 7,600 K. It is also believed that life can also exist on this particular type.
They are known for emitting a large amount of light in the form of radiation for example
UV Radiation which is harming our planet Earth. They are likely to represent the brightest
and hottest main-sequence stars that could possibly allow life. The best example of F-type
stars is Polaris i.e. the current Northern Pole star. A type F7 and is 430 light years
away.
Next up we have letter G: G type stars also called yellow dwarf are yellow in colour and
their weight is between 0.8 to 1.25 times the mass of the Sun. They are currently believed
to 100 light years away. Their Surface temperatures are between 5,300K and 6,000K. Class G contains
the Yellow Evolutionary Void i.e. it is an area in the Hertzsprung-Russell diagram where
atmospheres of blue-ward evolving super- and hyper-giants are moderately unstable. Thus,
this type is unstable for super-giants to be. Compared to hotter and brighter OBAF type
stars, G type stars radiate more light towards the infrared end of the spectrum. The most
famous G type, G2 type, star is Sun. Each second, the Sun fuses approximately 600 million
tons of hydrogen into helium in a process known as the proton-proton chain (4 hydrogens
form 1 helium), converting about 4 million tons of matter to energy. Apart from that
others known stars under this type is Alpha Centauri A, Tau Ceti, and 51 Pegasi.
Next is type K: K type stars appears orange and are cooler than the Sun. Their range of
temperature is between 3,500K to 6,000K. They are of intermediate size between M type and
G type. The keen interest astronomers is to research on this type to find life on this
star because of their stability, small mass, greater heat zones and they emit less UV rays.
Also the velocity of solar wind is not too much. Thus, they are the most approving stars
to focus on the research for ex-o-planets and extraterrestrial life. The nearest K-type
stars known are Epsilon Eridani which is 10.5 light years away, HD 192310 is 29 light years
away, Gliese 86 is 35 light years away, and 54 Piscium is 36 light year away.
And the last we have is type M: M type stars also known as red dwarfs are the smallest
and coolest red colour star with such low luminosity that none can be seen by our naked
eyes. Despite of being called as smallest they too include some giant, super giant stars
and hyper giant stars. The range of their surface temperature is from 2,400K to 3,700K.
Red dwarf stars have masses from about 0.08 to 0.6 times that of the Sun. Lighter stars
are more abundant in space than the heavier stars that is why they are the most common
and tends to live longer than usual.
There are other extended spectral types have been also included. A number of new spectral
types have been taken into use from newly discovered types of stars. This is a basic
video so we will not include these extended spectral types. But if you want to know more
about them let us know in the comment section and we will make a new video only on these
new spectral types. The basis of the current system of classification
of spectral types began in the late 1800s and is still going on and will carry for another
few hundred or maybe more years because of its boundless, far-reaching description. Several
spectral types, all previously used for non-standard stars in the mid-20th century, have been replaced
during revisions of the stellar classification system. They may still be found in old editions
of star catalogs: R and N have been subsumed into the new C class as C-R and C-N. Humans
may eventually be able to colonise any kind of stellar habitat, this section will address
the probability of life arising around other stars. Stability, luminosity, and lifespan
are all factors in stellar habitability. We only know of one star that hosts life, and
that is our own—a G class star with an abundance of heavy elements and low variability in brightness.
It is also unlike many stellar systems in that it only has one star in it. In the coming
future we hope to explore these endless boundaries of cosmos.
In this video we have discussed about the above spectral types and if you want to know
more about some of the extended types, let us know in the comment section. What do you
think about these spectral classes of stars? Did you know about them before? What did you
think about the video? Let us know in the comment section below. And don’t forget
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