Astronomy Notes

Astronomy

• Quote: "And yet it moves," - Galileo Galilee

• Astronomy is the study of the universe, celestial bodies, gas, and dust within it.

• Includes observations and theories about:

  • The solar system

  • Stars

  • Galaxies

  • The general structure of space

• Universe:

  • Vast empty space containing stars, solar systems, galaxies, etc.

  • Sum total of all matter and energy that exists or has existed in space and time.

  • "The totality of everything that exists, including all matter and energy, the planets, stars, galaxies and the contents of intergalactic space."

Astronomical System of Units

• Developed for use in astronomy.

• Adopted by the International Astronomical Union (IAU) in 1976; updated in 1994 and 2009.

• Developed to address difficulties in using International System of Units (SI units) for astronomical data.

• Tri-dimensional system: defines units of length, mass, and time.

Astronomical Unit of Time

• The day is defined as $86400$ seconds.

• $365.25$ days make up one Julian year.

• The symbol D represents this unit.

Astronomical Unit of Mass

• The solar mass is equal to $1.98892 \times 10^{30}$ kg.

• Standard unit to express mass in astronomy.

• Used to describe the masses of other stars and galaxies.

• Equal to the mass of the Sun.

Astronomical Units of Length

• Astronomical unit (AU) is the average distance between the Earth and the Sun.

• Approximately $150$ million kilometers.

• Exactly $149,597,870,700$ meters.

• Light-year and parsec are units for measuring huge distances.

• Light-year:

  • Distance light travels in one year in a vacuum.

  • About $5.8 \times 10^{12}$ miles.

  • Or $63,240$ AU.

  • Or $9.46053 \times 10^{12}$ kilometers.

• Parsec:

  • Equal to $3.26$ light-years.

  • Or $3.085678 \times 10^{13}$ kilometers.

  • Distance at which a star has a parallax of 1 second of arc.

Origin of the Universe

• Quote: "The problem of the origin of the universe is a bit like the old question: Which came first, the chicken, or the egg. In other words, what agency created the universe? And what created that agency? Or perhaps, the universe, or the agency that created it, existed forever, and didn't need to be created." - Stephen Hawking

• Big Bang Theory:

  • Leading explanation for the beginning of the universe.

  • The universe started with a small singularity.

  • Inflated over the next 13.7 billion years to the cosmos we know today.

  • Much understanding comes from mathematical theory and models due to limitations in peering back to the universe's birth.

  • Most astronomers believe the universe began in a Big Bang about $13.7$ billion years ago.

  • The entire universe was inside a bubble thousands of times smaller than a pinhead, named singularity.

  • It was extremely hot and dense.

  • Then it suddenly exploded, birthing time, space, and matter.

  • In a fraction of a second, it grew from smaller than an atom to bigger than a galaxy and continues to expand today.

• Nicolas Copernicus proposed a heliocentric (sun-centered) model for the solar system.

  • The Sun is stationary at the center and Earth and other planets orbit around it.

  • Before Copernican theory, Earth was believed to be stationary at the center with the Sun and planets revolving around it.

  • Galileo attempted to publicize Copernican theory in the early 1600s and was convicted as a result.

  • Copernican theory was not widely accepted until the late 17th century.

• As the universe expanded and cooled, energy changed into particles of matter and anti-matter.

  • These largely destroyed each other, but some matter survived.

  • More stable particles called protons and neutrons started to form when the universe was one second old.

  • Over the next three minutes, the temperature dropped below 1 billion degrees Celsius.

  • It was cool enough for protons and neutrons to come together, forming hydrogen and helium nuclei.

• After $300,000$ years, the universe cooled to about $3000$ degrees.

  • Atomic nuclei could finally capture electrons to form atoms.

  • The universe was filled with clouds of hydrogen and helium gas, which formed all celestial bodies afterwards.

Evidence for the Big Bang

• Expanding Universe:

  • By analyzing the spectrum of light from a star or galaxy, astronomers can tell its speed and direction.

  • Red Shift: Object moving away from Earth, light shifts to longer, redder wavelengths.

  • Blue Shift: Object moving toward Earth, light shifts to shorter, blue wavelengths.

  • Red shift supports the Big Bang theory.

  • Light from distant galaxies is red shifted, indicating they are moving away.

  • The greater the distance, the greater the red shift, indicating faster movement.

• Cosmic Background Microwave Radiation:

  • Arno Penzias and Wilson detected cosmic microwave background radiation while studying radio signals.

  • Radiations are received from all parts of the universe.

  • Thought to be the heat left over from the original expansion or Big Bang.

• Alternative Theories:

  • Some argue the Big Bang theory is based on a false premise.

  • Some prefer theories based on a static universe, originally predicted by Einstein's theory of general relativity.

The Ultimate Fate of the Universe

• Once the notion that the universe started with a rapid expansion nicknamed the Big Bang became accepted by the majority of scientists, the ultimate fate of the universe became a valid cosmological question, one depending upon the physical properties of the mass/energy in the universe, its average density, and the rate of expansion.

• Growing consensus among cosmologists that the universe is flat and will continue to expand forever.

• Dependent on the shape of the universe and the role of dark energy as the universe ages.

• Astronomers recognize four models of possible futures for the universe:

  • Open Model (Expanding Universe Theory)

  • Close Model (Contracting Universe Theory)

  • Flat Model (Steady State Theory)

  • Cyclic Model (Cyclic Universe Theory)

Galaxy

• A gravitationally bound system of stars, stellar remnants, interstellar gas, dust, and dark matter.

• Word galaxy derived from the Greek "galaxias," meaning "milky," a reference to the Milky Way.

• Astronomers estimate there are about 100 to 200 billion galaxies in the observable universe.

• Examples: Milky Way and Andromeda.

• Galaxies range in size from dwarfs with a few thousand stars to giants with one hundred trillion stars.

• Categorized by visual morphology: elliptical, spiral, and irregular.

• Many galaxies are thought to have black holes at their active centers.

• Black Hole: A massive object (or region) in space that is so dense that within a certain radius, its gravitational field does not let anything escape from it, not even light.

Milky Way

• The large, disk-shaped galaxy includes our solar system

• A spiral galaxy is shaped like a disk, usually with a bulge in the center and with arms that spiral outwards as the galaxy rotates.

• In addition to the Sun, the Milky Way contains about 200-400 billion other stars.

• Most of the stars that we can see are in the Milky Way Galaxy.

• Looks like a faint band of white in the night sky.

• Main features:

  1. About $100,000$ light years in diameter.

  2. Formed about 13.7 billion years ago.

  3. Sun takes roughly 250 million years to orbit around the Milky Way.

  4. Our solar system is about $30,000$ light years away from the center.

  5. Major arms: Pursues Arm, Sagittarius Arm, Centaurs Arm, and Cygnus Arm.

  6. Our Solar System is in a minor arm called the Orion Spur/Orion Arm.

  7. The region in space occupied by Milky Way is called Galactic Plane.

  8. Nearest galaxy to Milky Way Galaxy is Andromeda.

  9. Our Milky Way is the part of cluster of three dozens of galaxies called 'Local Group'.

Solar System

• Consists of the Sun, planets, dwarf planets, moons, an asteroid belt, comets, meteors, and other objects

• The Sun is the center and all other objects orbit it

• Elliptical (egg-shaped)

• The Sun contains more than $99\%$ of the solar system's mass.

• Includes:

  • Planets

  • Dwarf Planets

  • Satellites

  • Asteroids

  • Meteoroids

  • Comets

• Around $4.5$ billion years old

• Formed from a huge cloud of gas and dust called the solar nebula

• Under gravity, the cloud collapsed, forming the Sun and a disc of matter in which the planets were born

The Sun

• A star: A huge ball of gas, mostly hydrogen and helium.

• Nuclear fusion reactions inside release enormous energy.

• Very hot and emits its own light.

• Scientists divide it into three main regions: the interior, the solar atmosphere, and the photosphere (visible "surface") between them.

• Proxima Centauri is the closest star to Earth after the sun, $40$ trillion km away.

• Main parts of the Sun's Interior:

  • Core:

    • Center, hottest region.

    • Nuclear fusion reactions power the Sun.

    • Extends from the center to about $25\%$ of the solar radius.

    • Density is about $150$ times the density of water.

    • The only section of the sun that produces heat through fusion.

    • Temperature is $15$ million degrees Celsius.

  • Radiative (or radiation) Zone:

    • Energy is carried outward by photons as thermal radiation.

    • Material is hot and dense enough that thermal radiation (not fusion) transfers the intense heat of the Core outward.

  • Convective (or convection) Zone:

    • Heat moves through upward convection.

    • Plasma is not dense or hot enough for radiation to transfer heat.

    • Thermal convection occurs.

    • Cools at the surface, plunges downward to absorb more heat from the radiative zone, and repeats.

• The boundary between the Sun's interior and the solar atmosphere is called the photosphere.

  • What we see as the visible "surface".

  • Not like the surface of a planet.

• The lower region of the solar atmosphere is called the chromosphere.

  • From the Greek word chroma (meaning color), appears bright red during a solar eclipse.

• A thin transition region separates the chromosphere from the vast corona above, where temperatures rise sharply.

• The uppermost portion of the Sun's atmosphere is called the corona, and is surprisingly much hotter than the Sun's surface (photosphere).

Structure of the Sun

• Features include the Core, Radiation Zone, Convection Zone, Photosphere, Chromosphere, and Corona.

Important Physical Features of the Sun

1. Diameter: $1,391,940$ km ($870,000$ miles).

2. Mass: $1.989 \times 10^{30}$ kg or $330,000$ times that of Earth.

3. Composition: $74\%$ H2, $24\%$ He, and $2\%$ heavy metals (by mass).

4. Density: $1.41$ g/cm$^3$.

5. Temperature:

   • Core: $15$ million °C ($27.5$ million °F).

   • Surface: $5500$ °C ($9932$ °F).

6. Mean distance from Earth: $150$ million km or $93$ million miles.

7. Has a very large and active magnetic field.

8. Moves around the center of the galaxy at a velocity of $220$ km/s, revolving around the Milky Way in about $250$ million years.

9. Sun's rays reach Earth's surface in 8 minutes and 20 seconds.

Planets

• A body in orbit around the Sun.

• From the Greek word planetes, meaning "wanderers."

• Our solar system has eight planets: Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, and Neptune.

• Pluto is now classified as a dwarf planet.

• Mnemonic Device: My very educated mother just served us noodles (Mercury, Venus, Earth, Mass, Jupiter, Saturn Uranus and Neptune).

• Inner Planets: Mercury, Venus, Earth, and Mars.

  • Closest to the Sun.

  • Made up mostly of rock.

  • Rocky or terrestrial planets.

  • Very small compared to outer planets, therefore, also called inferior planets.

• Outer Planets: Jupiter, Saturn, Uranus, and Neptune.

  • Also called Jovian (Jupiter-like) planets because they are large in size.

  • Gaseous planets because they have an atmosphere of hydrogen and helium and they are have rings.

Terrestrial Planets

• Mercury, Venus, Earth, and Mars

• Close to Sun

• Small masses and radii

• Rocky, solid surfaces

• High densities

• Slow rotation

• Weak magnetic field

• No rings

• Few moons

Jovian Planets

• Jupiter, Saturn, Uranus, and Neptune

• Far from Sun

• Large masses and radii

• Gaseous surface

• Low densities

• Fast rotation

• Strong magnetic field

• Many rings

• Many moons

Physical Characteristics of Planets - individual descriptions

• Mercury

  1. Mercury is smallest and fastest revolving planet.

  2. Its diameter is 4900 km.

  3. Mercury is about the same size as our Moon.

  4. It is closest to the Sun.

  5. It was named after Mercury, the swift messenger of the gods in ancient Roman mythology.

  6. Mercury has no atmosphere, therefore, humans would not be able to live there.

  7. The surface of Mercury has holes in it where objects such as meteorites and asteroids crash into it

  8. It is made of dense rocky materials, therefore, has very high density.

  9. Mercury revolves around sun in 88 days

  10. It is 58 million km away from sun

  11. Its temperature varies from -170 to 450°C.

• Venus

  1. Venus is the hottest and brightest planet. Here temperature rises to 460-480 degrees Celsius on the side facing the Sun.

  2. Venus has very thick, rapidly spinning clouds of CO2 which cover its surface.

  3. These clouds hold heat in it that is why Venus gets so hot. These clouds also reflect sunlight that is why Venus appears so bright.

  4. Venus is almost equal the size of earth, Therefore, it is termed as sister planet of earth

  5. Venus has several large inactive volcanoes.

  6. Venus rotates from east to west while all other planets rotate from west and east.

  7. Venus spins very slowly. One day of Venus is equal to 243 earth days.

  8. Venus's revolution period is 225 days.

  9. It has no rings and no moons.

  • When Venus is visible in the morning, it reaches its maximum brightness shortly before sunrise, and when it is visible in the evening, it reaches its maximum brightness just after sunset that is why Venus got the names Morning Star and Evening Star.

• Earth

  1. Earth is the most colorful and the densest planet The Earth is the biggest of all the terrestrial planets.

  2. It is third in position and fifth largest in solar system.

  3. Venus is the closest planet to Earth.

  4. The diameter of earth is 12756 km or 7,926 miles.

  5. The circumference measured around the equator is 24,901 miles or 40,075 km.

  6. Earth is the only planet of solar system which can support life.

  7. About 30% of the Earth's surface is covered with land, while about 70% is covered by oceans.

  8. The Earth has one moon. Its name is Luna.

  9. It has an atmosphere made up of many different gases, mainly nitrogen and oxygen.

  10. Earth complete one revolution around sun in 365 days.

  11. The Earth rotates around its axis and it takes 23 hours 56 minutes and 4 seconds (one day) to spin around its axis.

  12. Earth is the densest planet; its density is 5.52 g/cm³.

• Mars

  1. Mars is the second smallest planet.

  2. Mars is called red planet due to the presence of red dust.

  3. The temperature on Mars can be very, very cold. At the top and bottom of the planet are poles just like on Earth.

  4. Mars has average temperature of -30°C.

  5. Mars surface is very interesting as it has many craters which were formed by meteorites or asteroids hitting it.

  6. Mars also has some of the tallest Mountains and some of the deepest valleys in our solar system.

  7. Mars has two moons, Phobos and Deimos, which have unusual shapes.

  8. Mars atmosphere is composed of CO2.

  9. Mars diameter is 6780 km.

  10. Mars is 228 million km away from sun.

  11. Its rotation period is 24 hours and 40 minutes.

  12. Its revolution period is 687 days.

• Jupiter

  1. Jupiter is the biggest and fastest rotating planet.

  2. Huge storms of swirling gases can be found in Jupiter's atmosphere.

  3. The largest storm of gas is called the Great Red Spot. Large bolts of lightning have also been seen in Jupiter's atmosphere.

  4. It is 1350 times bigger than earth in volume and 318 times massive than earth.

  5. Jupiter has 67 moons.

  6. Ganymede is the largest moon of Jupiter and is the largest in our solar system.

  7. One day of the Jupiter is equal to 9 hours and 55 minutes of the earth.

  8. Its revolution period is 11.86 years.

  9. It has rings which are not so visible.

  10. Average temperature of the Jupiter is -150°C.

• Saturn

  1. Saturn is the second biggest planet and is also called Ring Planet.

  2. Saturn is surrounded by rings made of ice and dust.

  3. Its rotation period is 10 hrs and 15 minutes and is second fastest rotating planet.

  4. Saturn is a very large gaseous planet which spins very rapidly on its axis.

  5. Saturn's atmosphere has winds which can blow at over 1800 kilometres per hour.

  6. The white spots on Saturn are believed to be powerful storms.

  7. Saturn has a density of 0.71 grams/cubic centimetre. Since Saturn is less dense than water, it would actually float like an apple if you could find a pool large enough.

  8. Saturn has 62 moons.

  9. Titan (Saturn's Moon) is the second largest moon of our solar system.

  10. Its diameter is 120,800 km.

  11. Its distance from sun is 1430 million km.

  12. Its average temperature is -180°C.

• Uranus

  1. Uranus is called Green Planet because its atmosphere is made up of hydrogen, helium and methane.

  2. The temperature in the upper atmosphere is very cold.

  3. The rapid rotation of Uranus causes winds up to 600 kilometres per hour to blow in its atmosphere.

  4. Uranus has 13 known rings which contain dark, boulder-sized particles.

  5. Uranus has 27 moons.

  6. Its rotation period is of 17 hours.

  7. Its revolution period is equal to 84 earth years.

  8. Its mean distance from sun is 2870 million km.

  9. Its diameter is 5180 km.

• Neptune

  1. Neptune is the farthest planet from the Sun.

  2. Neptune is the coldest and slowest revolving planet.

  3. Its orbital period is of 165 years.

  4. Neptune and Uranus are called twin planets.

  5. Neptune has winds in its atmosphere which blow at over 2000 kilometres per hour.

  6. This planet has large, dark circles on its surface which astronomers believe to be storms.

  7. Neptune has two thick and two thin rings surrounding it.

  8. Neptune also has 14 known moons.

  9. The period of rotation is about 16 hours.

  10. Its diameter is 49,400 km.

  11. It is 4500 million km away from sun.

  12. Its biggest moon Triton is the sixth largest moon of our solar system.

Difference between a Star and a Planet

Planets Physical Features Chart

Note: Figures are rounded off for the ease of students.

Dwarf Planets

• Defined by the International Astronomical Union (IAU):

  • In orbit around a star but not a satellite.

  • Sufficient mass to be nearly spherical.

  • Has not cleared the neighborhood around its orbit.

• Difference between a planet and a dwarf planet: whether it has cleared its orbit.

• Dwarf planets have not cleared their orbit due to small size and less gravitational pull.

• Five officially recognized dwarf planets in our solar system:

  • Ceres, Pluto, Haumea, Makemake, and Eris.

• Except for Ceres (in the asteroid belt) they are in the outer solar system in the Kuiper Belt.

• Pluto is unable to clear objects within the Kuiper Belt are within its orbit.

• For many years, Pluto was thought of as the farthest known planet from the Sun.

• Pluto has five moons.

Moon

• Natural satellite: A body in space that orbits a larger body (planet)

• The larger body is referred to as the satellite's primary

• Natural satellites that orbit planets are often called moons

• Our moon named 'Luna' is the fifth largest moon in the solar system.

  • Its diameter is 3475 km

  • Its mass is $7.3 \times 10^{22}$ kg ($1/81$ of earth)

  • Its density is 3.347 g/cm$^3$

  • The average distance of the Moon from the Earth is 384,400 kilometers (238,857 miles).

  • The Moon orbits the Earth every 27 days and 7 hours.

  • The effect of gravity on moon surface is one-sixth of earth's gravity.

  • The average surface temperature of the Moon is 107 degrees Celsius during the day and -153 degrees Celsius at night.

  • Moon has no atmosphere but it has negligible amount of inert gases.

  • It takes 1.3 seconds to moonlight to reach the earth.

  • Moon is one-fourth the size of its mother planet, i.e. earth.

Solar and Lunar Eclipse

• An eclipse takes place when one heavenly body such as a moon or planet moves into the shadow of another heavenly body.

Lunar Eclipse

• The moon moves in an orbit around Earth, and at the same time, Earth orbits the sun.

• Sometimes Earth moves between the sun and the moon.

• When this happens, Earth blocks the sunlight that normally is reflected by the moon. (This sunlight is what causes the moon to shine.)

• Instead of light hitting the moon's surface, Earth's shadow falls on it.

• This is an eclipse of the moon a lunar eclipse.

• A lunar eclipse can occur only when the moon is full.

• A lunar eclipse can be seen from Earth at night.

• The earth casts a long, conical shadow in space.

• At any point within that cone the light of the sun is wholly obscured.

• Surrounding the shadow cone, also called the umbra, is an area of partial shadow called the penumbra.

• A lunar eclipse usually lasts for a few hours.

• At least two partial lunar eclipses happen every year, but total lunar eclipses are rare.

• It is safe to look at a lunar eclipse.

Three Types of Lunar Eclipses

• Penumbral lunar eclipse - the Moon only passes through the penumbra of Earth's shadow. It is rarely visible from earth as there is slight change of color of the moon.

• Partial lunar eclipse - when part of the Moon passes through the umbra of Earth's shadow then it is termed as partial lunar eclipse as its whole area is not obscured by the shadow.

• Total lunar eclipse - when the entire Moon passes through the umbral region of Earth's shadow and moon is totally obscured.

Solar Eclipse

• Sometimes when the moon orbits Earth, it moves between the sun and Earth.

• When this happens, the moon blocks the light of the sun from reaching Earth.

• This causes an eclipse of the sun, or solar eclipse.

• During a solar eclipse, the moon casts a shadow onto Earth.

• This shadow on the Earth consists of two regions: the umbra and the penumbra.

• Solar eclipses happen once every 18 months.

• Unlike lunar eclipses, solar eclipses only last for a few minutes.

• Solar eclipse always occurs at the time of new moon.

• NEVER look directly at the sun: It can permanently damage your eyes! You must use proper safety equipment to look at any type of solar eclipse.

Types of Solar Eclipse

• A total solar eclipse occurs when the Moon completely covers the Sun, as seen from Earth. Totality during such an eclipse can only be seen from a very small area on Earth. This area is usually about 100 miles (160 km) wide and 10,000 miles (16,100 km) long. Areas outside this track may be able to see a partial eclipse of the Sun. Looking at a solar eclipse without any protective eyewear can severely harm one's eyes.

• A partial solar eclipse can be observed when the Earth, Moon and Sun do not align in a perfectly straight line, and the Moon only partially covers the disc of the Sun.

• An annular solar eclipse occurs when the Moon appears smaller than the Sun as it passes centrally across the solar disk and a bright ring, or annulus, of sun remains visible during the eclipse.

Why don't we have a lunar eclipse every month?

• The moon goes around Earth every month, but it doesn't always get in Earth's shadow.

• The moon's path around Earth is tilted compared to Earth's orbit around the sun. The moon can be behind Earth but still get hit by light from the sun.

Asteroids and Meteoroids

• Asteroids: the largest non-planetary and non-lunar objects in the solar system.

  • Larger than $100$ meters in diameter (IAU has not defined any size dimensions).

  • Less than $1000$ km in diameter.

  • The largest asteroid is Ceres, with a diameter of $950$ km.

  • Generally irregular in shape and often have surfaces covered with craters.

  • Also named as minor planets or Planetoids.

  • Most orbit between the orbits of Mars and Jupiter.

• Meteoroid: a small body travelling through space

  • Typically originate from comets and asteroids.

  • No well-defined size range for a meteoroid.

  • A piece of matter, when it strikes the Earth's atmosphere, produces the visual phenomenon called a meteor.

  • A fragment that survives to hit the ground is known as a meteorite.

Comet

• A celestial body that orbits around the sun.

• Made up of:

  • Nucleus (solid, frozen ice, gas, and dust)

  • Gaseous coma (water vapor, CO2 and other gases)

  • Tail (dust and ionized gases)

• Its long tail of gas and dust always points away from the sun, because of the force of the solar wind.

• The tail can be up to $250$ million km long.

• Comets are only visible when they're near the sun in their highly eccentric orbits.

• When comets are farthest from sun, they are named dirty snowball.

• Halley's Comet:

  • A periodic comet that orbits around the sun.

  • Edmund Halley was the first person to recognize that it is periodic.

  • It was last seen in 1986 and will be seen next in the year 2061; its period is 75 years.

Constellation

• Groups of stars visible within a particular region of the night sky forming various shapes and patterns.

• Named after the names of animals, tools, objects, religious personalities, and scientists.

• Viewable after sunset and before sunrise.

• There are a total of 88 named constellations.

• The largest one is Hydra.

• Other examples include Ursa Major,