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The celestial sphere is an imaginary sphere of arbitrarily large radius, concentric with a particular

celestial body.

Allows observers to plot positions of objects in the sky when their distances are unknown or

unimportant.

Right ascension is like longitude (how far east or west as from prime meridian)

Declination is like latitude (how far north or south of the earth’s equator)

Constellations are patterns formed by prominent stars within apparent proximity to one another on

Earth's night sky. These stars appear close to each other on the sky, but can be really far apart in

space.

e.g Leo, Virgo, Orion, Scorpius, Pegasus

1.Stars twinkle whereas planets do not (if atmospheric conditions are good).

2.Stars remain in the same position relative to each other, whereas planets do not (planets follow the

general path of the ecliptic, the imaginary path that the sun follows across the sky from east to west)

3. A star will be at the exact same position relative to the observer if observed at the same sidereal

time each time.

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Achievements; 1) built the first high-powered astronomical

telescope. 2) Presented the heliocentric model of the solar system. 3) Showed that velocities of

falling bodies are not proportional to their weights.

In the 1761, Alexandre Guy Pingre – visited Rodrigue to observe the transit of Venus.

came to MRU in 1874 to observe the transit of Venus and to determine the solar parallax.

The Mauritius Radio Telescope (MRT) was used to image of the sky at a frequency of 151.5 MHz. Its

resolution is about 4 arc min. The MRT is a T-shaped array consisting of fixed helical antennas.

It has also been used for pulsar observations. The MRT is also meant to map our galaxy, the Milky

Way. In addition, data on solar flares has also been collected.

In 1619, German astronomer Johannes Kepler figured out the distances of all the planets from the

Sun. Eg Mar’s is at 1.5 AU, Venus is at 0.72 AU.

But no one knew the the value of an AU. In 1716, English astronomer Edmond Halley used the transit

of Venus to find the AU.

At Bell Telephone Laboratories CarlJansky built an antenna designed to receive radio waves. After

recording signals from all directions for several months, Jansky detected a faint steady hiss of

unknown origin. Jansky determined that the signal repeated itself once each sidereal day, instead of

the 24-hour solar day. By comparing his observations with optical astronomical maps, Jansky

concluded that the radiation was coming from the Milky Way and was strongest in the direction of the

center the galaxy, in the constellation of Sagittarius.

The atmospheric window is the range of frequencies of electromagnetic radiation that the earth's

atmosphere lets through. (3 types :radio, infra-red, optical).

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1.Can see objects not visible in optical wavelengths

2.Can see through atmospheric and interstellar clouds.

3. Allowed discovery of quasars, CMBR, pulsars, radio galaxies.

4.Can be used to monitor solar activity

The (radio telescope in development in Australia and South Africa)

1. Total collecting area of approximately 1 km2.

2.Operate over a wide range of frequencies.

3. 50 times more sensitive than any other radio instrument (due to its size)

4. Able to survey the sky more than 10,000 faster than ever before.

5.Provide the highest resolution images in all astronomy.

6.Built in the southern hemisphere because the view of the Milky Way Galaxy is best and radio

interference least.

Construction of the SKA begins in 2018, initial observations by 2020, full operation by 2025.

Very-long-baseline interferometry (VLBI) is a type of astronomical interferometry used in radio

astronomy. Interferometry is a technique in which electromagnetic waves are superimposed in order

to extract information about the waves.

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1.Can observe in all wavelengths of EM radiation – not limited by the atmosphere.

2. Not dependent on atmospheric conditions (turbulence/bad weather does not prevent observations

from being made)

3.Can detect light coming from further away in the Universe (and hence detect objects which are

billions of light years away)

4.No distortion of images due to refraction by the atmosphere.

1.Probes have to send in outer space. Very Expensive

2.Probes have to be mostly autonomous thus very complicated and costly engineering.

3.The ability to improvise is very limited

4.Damaged to probe can be terminal, failing the whole mission irreversibly.

The is a space telescope that was carried into orbit by a Space Shuttle in

1990. It is still operational and it orbits the Earth. Hubble's four main instruments observe in the near

ultraviolet, visible, and near infrared spectra.

Observing celestial bodies at different wavelengths allows astronomers to gather huge amounts of

information about the structure and composition of these bodies. Multi-wavelength observations

allows objects emitting radiation of any wavelength to be detected. If we limit observations to a

particular range of frequencies, it may cause certain bodies (or certain features of a body) to go

undetected.

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Our solar system, estimated to begun 4.6 billion years ago due to the gravitational collapse of a giant

molecular cloud. Most collapsed to form the sun while the rest formed planets. Eventually the sun

became so massive that it gravitational collapse produced temperature hot enough to sustain nuclear

fusion, thus igniting the sun which produced a shock wave and solar wind which pushed most gases

away from the sun thus forming gas giants like Jupiter, Neptune … while rocky planets like earth

remained relatively close to the sun

Sun,Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, Neptune.

All planets orbit the sun in a slightly elliptical orbits (ie it is almost circular). Mercury, however has a

very elliptical orbit.

The inner planets are closer to the Sun and are smaller and rockier. Called terrestrial planets because their surfaces are solid .They’re made up mostly of heavy metals such as iron and nickel, and have either no moons or few moons. Mercury: Smallest planet in Solar System. No moons, very thin atmosphere (O2,Na,H2,He,K) Venus: Surface temperature of 480 º C. Thick atmosphere – CO2 and N2. No rings or moons. Earth: Earth is the only planet with life as we know it. Atmosphere of N2 and O2. 1 moon and no rings. Mars: Mars shows signs of liquid water flowing on its surface in the ancient past. Atmosphere - Ar,CO2 and N2. 2 tiny moons (Phobos and Deimos) and no rings. A Mars day is slightly longer than 24 Earth hours.

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Despite their size, only two of them are visible without telescopes: Jupiter and Saturn. Uranus and Neptune were the first planets discovered since antiquity, and showed astronomers the solar system was bigger than previously thought. Jupiter: Jupiter is the largest planet in our Solar System. Spins very rapidly (10 Earth hours) relative to its orbit of the sun (12 Earth years). Its thick atmosphere (H2 and He). Possible terrestrial core about Earth’s size. Dozens of moons. Faint rings. Great Red Spot — a raging storm happening for the past 400 years at least. Saturn: Prominent ring system — seven known rings. Dozens of moons. Atmosphere - mostly H2 and He. Rotates quickly (10.7 Earth hours) relative to its time to circle the Sun (29 Earth years). Uranus: Discovered by William Herschel.Day. Contains water, NH3,H2,He and methane. Rocky core. Dozens of moons. Faint ring system. Neptune: Distant planet that contains water, NH3,H2,He. Possible Earth-sized core. More than 12 moons and 6 rings.

Feature Inner planets Outer planets

Size Small Large

Solid surfaces Present Balls of gas (might have solid/liquid core)

Density High Low

Atmospheres Varied Similar

Rotation about its own axis Slow Fast

Orbit around the Sun Fast Slow

Rings None All (Saturn's ones most prominent)

Moons Very few Many (63 around Jupiter)

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The moon’s gravitational force has a considerable effect on our earth. Its most visible one is the high

and low tides of earth’s seas produced by the gravitational pull of the moon. The moon takes about

29.5 earth days to rotate around our earth. The Moon’s surface has a lot of craters as a result of

asteroids and comets colliding with the lunar surface.

An eclipse is an astronomical event that occurs when an astronomical object is temporarily obscured, either by passing into the shadow of another body or by having another body pass between it and the viewer. An eclipse involving the Sun, Earth and Moon can occur only when they are nearly in a straight line, allowing one body to be hidden behind another, when viewed from the third body.

Occurs at New Moon when the Moon passes between Earth and Sun. The Moon's shadow actually has two parts: 1. Penumbra (The Moon's faint outer shadow. Partial solar eclipses are visible from within the penumbral shadow.) 2. Umbra (The Moon's dark inner shadow)

1.Total solar eclipses: visible from within the umbral shadow. 2.Partial solar eclipse: visible from within the penumbral shadow 3.Annular solar eclipses Why do annular eclipses occur? The Moon's orbit around Earth is elliptical in shape, hence its distance from Earth varies and so does the Moon's apparent size. When the Moon is on the near side of its orbit, the Moon appears larger than the Sun - total solar eclipse. Moon on the far side of its orbit, the Moon appears smaller than the Sun and can't completely cover it. Moon is too small to cover the entire Sun's disk – annular solar eclipse

Moon passes through Earth's shadow. Occurs when the Moon is in the Full Moon phase. outer or penumbral shadow: zone where the Earth blocks part but not all of the Sun's rays from reaching the Moon. inner or umbral shadow: a region where the Earth blocks all direct sunlight from reaching the Moon.

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1. Penumbral Lunar Eclipse The Moon passes through Earth's penumbral shadow (difficult to observe) 2. Partial Lunar Eclipse A portion of the Moon passes through Earth's umbral shadow. (Can be seen with naked eye) 3. Total Lunar Eclipse The entire Moon passes through Earth's umbral shadow. (Moon appears red)

An Aurora is a natural display of light in the sky. This is a result of the ions flowing outward from the

sun (solar wind), the earth’s magnetic field traps these particles, many of which travels toward the

poles. Collision between these ions and the atmospheric atoms/molecules makes the latter ionized or

excited. Upon de-excitation visible light is emitted which we see as the aurora.

Habitable zone is the orbital region around a star in which an Earth-like planet has a suitable

temperature and atmospheric pressure such that liquid water can exist on its surface and possibly

support life.

Mars is more similar to Earth than anywhere else in the solar system, BUT:

No magnetic field (no protection from harmful space radiation)

Very thin atmosphere

Frozen water present in polar caps and beneath the surface but temperature and pressure at surface

too low for liquid water to exist.

Mars may have hosted life in the past - About 3.8 billion years ago, there was a denser atmosphere,

higher temperature, and vast amounts of liquid water flowed on the surface (evidence: gullies, ocean

beds, signs of erosion)

Curiosity rover currently searching for signs of ancient life on Mars.

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Asteroids: large chunks of rock that come from the asteroid belt located between the orbits of Mars

and Jupiter. Sometimes their orbits get perturbed / altered and some asteroids end up coming closer

to the Sun, and therefore closer to Earth.

Comets: similar to asteroids, but might have more ice, methane, ammonia, and other compounds that

cause it to develop a fuzzy, cloud-like shell called a coma – as well as a tail — when it gets closer to

the Sun. Comets are thought to originate from two different sources: Long-period comets (those

taking > 200 years to complete an orbit around the Sun) originate from the Oort Cloud. Short-period

comets (those which take < 200 years to complete an orbit around the Sun) originate from the Kuiper

Belt.

Meteoroid: Space debris smaller than an asteroid. A piece of interplanetary matter that is smaller than

a km and frequently only mm in size. Most meteoroids that enter the Earth’s atmosphere are so small

that they vaporize completely and never reach the planet’s surface.

Meteors. Another name commonly used for a meteor is a shooting star. A meteor is the flash of light

that we see in the night sky when a small chunk of interplanetary debris burns up as it passes through

our atmosphere. “Meteor” refers to the flash of light caused by the debris, not the debris itself.

Meteorite: If any part of a meteoroid survives the fall through the atmosphere and lands on Earth, it is

called a meteorite. Although the vast majority of meteorites are very small, their size can range from

about a fraction of a gram (the size of a pebble) to 100 kilograms or more (the size of a huge, life-

destroying boulder).

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1.Huge asteroid falling on earth can wipe out life. The impact will make a hole several miles in

diameter in earth’s crust and matter will be projected high above the atmosphere. The matter falls

back on earth causing further damage. At the impact, extreme temperatures exist, which will travel

across the planet evaporating the seas. No living thing is known to survive such extreme

temperatures.

2. Gamma ray burst from a supernovas can hit the earth. The gamma ray would destroy our ozone

layer, making earth vulnerable to UV from the sun . The gamma ray would irradiate all living thing and

killing them.

3. Strolling black holes can swallow the whole solar system.

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The Oort cloud is an immense spherical cloud surrounding the planetary system and extending from

5000 – 100,000 AU. This vast distance is considered the edge of the Sun's orb of gravitational

influence.

Within the cloud, comets are typically tens of 106 of km apart. They are weakly bound to the sun, and

passing stars and other forces can readily change their orbits, sending them into the inner solar

system or out to interstellar space..

Most comets are thought to originate from the Oort Cloud. Although its existence has not yet been

proven through direct observation, the reality of the Oort Cloud is widely accepted in the scientific

community.

The Kuiper Belt is a doughnut-shaped ring just beyond the orbit of Neptune. It is similar to an asteroid

belt, but while the asteroid belt is mostly metal and rock, the Kuiper Belt is composed almost entirely

of icy chunks of frozen water, ammonia and various hydrocarbons (same like comets).

There may be are hundreds of thousands of icy bodies larger than 100 km and an estimated trillion or

more comets within the Kuiper Belt. Some of these Kuiper Belt objects are massive (such as the

dwarf planet Pluto). Several dwarf planets in the Kuiper Belt have tiny moons.

The Kuiper Belt can be divided into smaller sections some of which are mostly unaffected by

Neptune’s gravitational effect, so the objects there can remain stable in their orbits.

Short-period comets: orbits ~200 years, come from the Kuiper belt

Long-period comets: orbits ~1000 years, come from Oort Cloud

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Solar time is based on the position of the Sun in the sky. There are 2 types of solar time:

Apparent solar time (indicated by a sundial) and mean solar time (shown by a common clock).

Apparent Solar Time

Apparent solar time or true solar time is based on the apparent motion of the actual Sun. It is based

on the apparent solar day (whose length of which varies during a year), the interval between two

successive returns of the Sun to the local meridian.

Thus sundials measure time based on the actual position of the Sun in the local sky. Noon is the precise moment when the Sun is on the meridian (an imaginary line passing from the north to south through the zenith) and the sundial casts its shortest shadow. Before noon, when the Sun is on its way to meridian, the apparent solar time is ante meridian (a.m.) and past noon the apparent solar time is post meridian (p.m.). Mean Solar Time Even though the average solar day is 24 hours, the actual length of the solar day varies throughout the year. Thus a watch will not remain perfectly synchronized with the sundial over the year. Hence it is more convenient to define an average of the apparent solar time. This is the mean solar time and is the basis of standard time. The value of the difference between mean and apparent solar time is called the Equation of Time (see below). Mean solar time makes use of the concept of a 'mean sun': an imaginary sun which matches a mean

solar day exactly. It orbits the Earth at a constant rate so that every day is the same length.

Thus the mean solar time is given by the hour angle of the mean Sun plus 12 hours. Mean solar time

would be the time indicated by a steady clock set so that over the year its differences from apparent

solar time average to zero.

The Equation of Time Universal time (clock time) is the mean solar time. An apparent solar day can be 20 seconds shorter

or 30 seconds longer than a mean solar day. The equation of time is the difference between the

mean solar time and the apparent solar time:

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Mean Solar Time = Apparent Solar Time + equation of time

NOTE: A longitude correction is required to translate local apparent solar time (L.A.T.) to the L.A.T.

for the central meridian for our particular time zone. Hence:

Mean Solar Time = Apparent Solar Time + (equation of time - 4*longitude + 60*time zone)

For Mauritius: longitude = 57.5 º (East)

time zone= + 4 (relative to Coordinated Universal Time)

The graph of the equation of time is closely approximated by the sum of two sine curves which reflect

the following 2 effects, each of which contribute to the non-uniformity in the apparent daily motion of

the Sun relative to the stars:

1. The eccentricity of the Earth's orbit around the Sun On Earth, the Sun appears to revolve once around the Earth through the background stars in one

year. If the Earth orbited the Sun with a constant speed, in a circular orbit in a plane perpendicular to

the Earth's axis, then the Sun would culminate every day at exactly the same time, and be a perfect

time keeper (except for the very small effect of the slowing rotation of the Earth).

But the orbit of the Earth is an ellipse which is not centered on the Sun (as according to Kepler's laws

of planetary motion) and its speed and angular momentum vary, and thus the Sun appears to move

faster (relative to the background stars) at perihelion and slower at aphelion.

As a result the eccentricity of the Earth's orbit contributes a sine wave variation with amplitude of 7.66

minutes and a period of one year to the equation of time.

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2. The obliquity of the ecliptic Even if the Earth's orbit were circular, the perceived motion of the Sun along our celestial equator

would still be non-uniform due to the tilt of the Earth's rotational axis with respect to the plane of its

orbit, or equivalently, the tilt of the ecliptic (the path of the Sun seems to take in the celestial sphere)

with respect to the celestial equator.

An illustration of obliquity is that the daily shift of the shadow cast by the Sun in a sundial even on the

equator is smaller close to the equinoxes and greater close to the solstices.

In terms of the equation of time, the inclination of the ecliptic results in the contribution of a sine wave

variation with an amplitude of 9.87 minutes and a period of a half year to the equation of time. Above

the horizontal axis, the sundial is 'fast' relative to a clock showing local mean time, and thus the value

of the equation of time must be subtracted from the apparent solar time.

Thus it can be seen that the equation of time would be constant only for a planet having zero axial tilt and zero orbital eccentricity.

Sidereal time is a time scale that is based on the Earth's rate of rotation measured relative to the fixed

stars(instead of the Sun). Hence, from a given observation point, a star found at one location in the

sky will be found at nearly the same location on another night at the same sidereal time.

Because the Earth moves in its orbit around the Sun, the Earth must rotate more than 360 degrees

(actually 360.986 º) in 1 solar day.

But since the stars are so distant from us, the effect of motion of the Earth in its orbit on the apparent

positions of the stars is negligible. Hence, the Earth rotates exactly 360 º in one sidereal day, making

one sidereal day last for about 23 h 56 m 4.1 s.

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A sidereal day lasts from when a distant star is on the meridian at a point on Earth until it is again at

the same point on the meridian.

Our sun was formed about 4.6 billion years ago from a gravitational collapse of large molecular cloud.

The central mass became increasingly hot and dense, eventually initiating thermonuclear fusion in its

core. The Sun is a G-Type (surface temperature 6000k) and it is actually white in color. From the

surface of the Earth it may appear yellow because of atmospheric scattering of blue light. The mean

distance of the Sun from the Earth is approximately 1 astronomic unit ( 1.5 × 10 ). Light from the sun

takes 8.3 minutes to reach earth. Sun’s core is about 15.7 million Kelvin. The Sun is about 75%

hydrogen and 23.8% helium and few other elements such as carbon, oxygen. The Sun is

magnetically active star and the changing magnetic field varies from year to year and reverses

direction about every eleven years around solar maximum. A Solar flare is a sudden flash of

brightness observed over the Sun’s surface. The flare ejects clouds of electrons, ions, and atoms

through the corona of the sun into space. The solar wind is a stream of plasma released from the

upper atmosphere of the Sun. It consists of mostly electrons and protons. The solar wind flows

outward supersonically to great distances, filling a region known as the heliosphere, an enormous

bubble like volume surrounded by the interstellar medium.

The sun today is roughly halfway through the most stable part of its life. The hydrogen powering the

sun’s nuclear fusion will eventually run out, causing the core of the sun to crush on itself, making it

even hotter. As a result, the Sun expands and become a red giant. Since it expands, the surface

temperature is a lot cooler. The luminosity will increases dramatically because of its larger surface

area. Our Sun is not massive enough to sustain Helium nuclear fusion, thus the Red giant will

eventually collapse on itself to become a white dwarf and after trillions of years, and it will become a

black dwarf. If the sun was 3 or more times heavier, it would have become a black hole instead a

white dwarf.

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Mass M: 0.05 to 100 M⊙, deduced from application of Keplers' Laws to binary systems of stars. Luminosity L: Energy radiated per sec from the surface, deduced from measurements of the Flux received, and the distance (if known). L varies from 10−6 to 106 L⊙. Effective Temperature Te: surface temperature, equivalent to that of a Black body:

L = σ4πR2T4

Deduced from spectroscopy.

In the spectral classification of stars (O B A F G K M) : [Te 30,000 -3,000 K]

Radii R may be deduced if L and Te are known, or from interferometry. Can be 10−3 to 103 R⊙.

Colour Index: Used to determine the colour of an object, which in the case of a star gives its temperature. To measure the index, one observes the magnitude of an object successively through two different filters. The smaller the color index, the bluer (or hotter) the object is. Conversely, the larger the color index, the more red (or cooler) the object is. U: Ultraviolet B: Blue V: Visible U-B or B-V often used, since stars have a blackbody spectrum (roughly)

A Star is essentially a big sphere (say of radius R) of hot gas composed mainly of hydrogen. It is able

to maintain a stable size if there is an exact balance between the inward force due to gravity and the

outward force due to a pressure gradient in the gas (i.e hydrostatic equilibrium)

( ) = −휌(푟)푔(푟)

where P(r), ρ(r) and g(r) are the pressure, density and acceleration due to gravity at a radial distance

r from the centre of the star.

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Parallax method :

If you hold your finger in front of your face and close one eye and look with the other, then switch eyes, you'll see your finger seem to "shift " with respect to more distant objects behind it. The effect is called parallax.

Astronomers can measure parallax by measuring the position of a nearby star very carefully with respect to more distant stars behind it, then measuring those distances again six months later when the Earth is on the opposite side of its orbit. From this, the distance in parsecs or other units can be calculated.

Inverse-square law:

The apparent brightness of a star depends both on its luminosity and its distance from us. If we know the luminosity of a star, we can measure its apparent magnitude and work out the distance using the inverse-square law.

푑(푝푎푟푠푒푐푠) = 1

푝 (푎푟푐푠푒푐)

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퐷푖푠푡푎푛푐푒푚표푑푢푙푢푠푒푞푢푎푡푖표푛 푚 −푀 = 5.0푙표푔푑 − 5.0

푷풐품풔풐풏 풔푹풆풍풂풕풊풐풏

풎ퟏ −풎ퟐ = −ퟐ.ퟓ풍풐품푭ퟏ푭ퟐ

푹풆풔풐풍풖풕풊풐풏풐풇풕풆풍풆풔풄풐풑풆휽 = 흀풅

흀:풘풂풗풆풍풆풏품풕풉 풅:풅풊풂풎풆풕풆풓풐풇풕풆풍풆풔풄풐풑풆

퓥풓 = 푯풅 퓥풓:풓풆풄풆풔풔풊풐풏풂풍풗풆풍풐풄풊풕풚 푯:푯풖풃풃풍풆풄풐풏풔풕풂풏풕 풅:풅풊풔풕풂풏풄풆 푫풐풑풑풍풆풓풆풇풇풆풄풕 − 풓풆풅풔풉풊풇풕 ∆흀흀

=퓥풓풄

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푷풓풆풔풔풖풓풆푷풄풂풕풕풉풆풄풆풏풕풓풆풐풇풕풉풆풔풕풂풓

푷풄 ∝ 푴ퟐ

푹ퟒ

푴ퟐ:푴풂풔풔풐풇풔풕풂풓

푹ퟒ:푹풂풅풊풖풔풐풇풔풕풂풓

푇푒푚푝푒푟푎푡푢푟푒푎푡푡ℎ푒푐푒푛푡푟푒표푓푠푡푎푟

푃 = 푛퐾푇 = kT

푚 :푚푎푠푠표푓ℎ푦푑푟표푔푒푛 휇 = 0.5푖푓푤푒푎푠푠푢푚푒푓푢푙푙푦푖표푛푖푧푒푑ℎ푦푑푟표푔푒푛 휌:푑푒푛푠푖푡푦푎푡푡ℎ푒푐푒푛푡푒푟

퐶푒푛푡푟푎푙푇푒푚푝푒푟푎푡푢푟푒푇

푇 ∝푀푅

A supernova is a stellar explosion that are extremely luminous and cause a burst of radiation that

often briefly outshines the entire, before fading from view over several weeks or months. Supernovae

can be triggered by the gravitational collapse of the core of a massive star.

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A planetary nebula is a kind of emission nebula consisting of an expanding glowing shell of ionized

gas ejected from old red giant stars late in their life.

Interstellar medium is the matter that exists in the space between the star systems in a galaxy. It

consists of gas in ionic, atomic and molecular form, dust and cosmic rays.

A neutron star is a type of stellar remnant that can result from the gravitational collapsed of a massive

star during a supernova event. Neutron stars are the densest and tiniest stars known to exist in the

universe.

A pulsar is a highly magnetized, rotating neutron star that emits a beam of electromagnetic radiation.

A black hole is defined as a region of spacetime from which gravity prevents anything, including light,

from escaping, The theory of general relativity predicts that a sufficiently compact mass will deform

space-time to form a black hole.

Main sequence star: stars that are fusing hydrogen atoms to form helium atoms in their cores. Most of

the stars in the universe — about 90 percent of them — are main sequence stars. The sun is a main

sequence star.

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Once the Sun has become a red giant, it will still be eating up He and cranking out C. But when it's finished its He, it isn't quite hot enough to be able to burn the carbon it created, and will thus succumb to gravity again. When the core of the star contracts, it will cause a release of energy that makes the envelope of the star expand. Now the star has become an even bigger giant than before!

The Sun will not be very stable at this point and will lose mass. This continues until the star finally blows its outer layers off. The core of the star, however, remains intact, and becomes a white dwarf. The white dwarf will be surrounded by an expanding shell of gas in an object known as a planetary nebula.

A low or medium mass star (<8 Mʘ) will become a white dwarf. A typical white dwarf is about as massive as the Sun, yet only slightly bigger than the Earth (hence very dense).

Galaxies in our universe seem to be rotating with such speed that the gravity generated by their observable matter could not possibly hold them together; they should have torn themselves apart long ago. The same is true of galaxies in clusters. Something we have yet to detect directly is giving these galaxies extra mass, generating the extra gravity they need to stay intact. This strange and unknown matter was called “dark matter” since it is not visible.

Unlike normal matter, dark matter does not interact with the electromagnetic force. This means it does not absorb, reflect or emit light, making it extremely hard to spot. Researchers have been able to infer the existence of dark matter only from the gravitational effect it seems to have on visible matter. Dark matter makes up about 26% of all the matter in the universe. The matter we know and that makes up all stars and galaxies only accounts for 4% of the content of the universe!

Dark energy makes up approximately 70% of the universe and appears to be associated with the vacuum in space. It is distributed evenly throughout the universe, not only in space but also in time –

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in other words, its effect is not diluted as the universe expands. The even distribution means that dark energy does not have any local gravitational effects, but rather a global effect on the universe as a whole. This leads to a repulsive force, which tends to accelerate the expansion of the universe. The rate of expansion and its acceleration can be measured by observations based on the Hubble law. These measurements, together with other scientific data, have confirmed the existence of dark energy and provide an estimate of just how much of this mysterious substance exists.

A barred spiral galaxy

1. Galactic disk: Most of the stars located here.The disk is made of old and young stars, as well as vast amounts of gas and dust. Stars within the disk orbit the galactic center in roughly circular orbits. (Gravitational interactions between the stars cause the circular motions to have some up-and-down motion, like horses on a merry-go-round).

The disk itself is broken up into these parts:

Nucleus: The center of the disk

Bulge: This is the area around the nucleus, including the immediate areas above and below the plane of the disk.

Spiral arms: These areas extend outward from the center. Our solar system is located in one of the spiral arms of the Milky Way.

2. Globular clusters: Scattered above and below the plane of the disk. Globular clusters orbit the galactic center in elliptical orbits in which the directions are randomly scattered. The stars in the globular clusters are much older stars than those in the galactic disk, and there's little or no gas and dust.

3. Halo: This is a large, dim, region that surrounds the entire galaxy. The halo is made of hot gas and possibly dark matter.

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1.Spiral galaxies:

Spiral galaxies are galaxies whose visible stars take on a spiraling pinwheel shape. The stars and

other visible material contained in such a galaxy lie mostly on a plane. Most of the galaxies in the

universe observed by scientists are spiral galaxies. These galaxies are made up of hot young stars.

2.Elliptical galaxies

These galaxies have an ellipsoidal profile, giving them an elliptical appearance regardless of the

viewing angle. Elliptical galaxies have very little gas and dust. Since stars form from gas, little star

formation occurs in elliptical galaxies. Most of their stars are old and red.

The effective temperature of the star (sometimes called the surface temperature) is the temperature of a black body having the same size and luminosity as the star and is determined by Stefan's Law. The variations in spectral lines for different stars are due primarily to the difference in temperature of the outer layers of gas in the star.

The standard spectral class classification scheme is based on the effective temperature. Most stars fit into one of the following types or spectral classes:

O:hottest

M:coolest.

Plot of the absolute magnitude of stars against their colour (hence effective temperature) or spectral

class against absolute magnitude:

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