Solar System and Key Terms

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1. International Astronomical Union (IAU), Paris, France

• It acts as the internationally recognized authority for assigning designations and names to celestial bodies (stars, planets, asteroids, etc.); and any surface features on them.
• International association of professional astronomers, at the PhD level and beyond, active in professional research and education in astronomy.
• It is a member of the International Science Council (ISC).
• India is a member.

Important bodies:

• Minor Planet Centre: Clearinghouse for all non-planetary or non-moon bodies in the Solar System.
• Working Group for Planetary System Nomenclature (WGPSN):  maintains the astronomical naming conventions and planetary nomenclature.
• Working Group on Star Names (WGSN): catalogues and standardizes proper names for stars.

2. Celestial bodies/ Astronomical object

• Any naturally occurring body outside Earth's atmosphere.

3 types:

• Stars: Own heat and light.
• Planets: no heat or light.
• Moons/ Satellites.

Planets size:

• Large to small: Jupiter, Saturn, Uranus, Neptune, Earth, Venus, Mars, and Mercury.
• Diameter of Earth: 12,742 km.
• Diameter of Sun- 864,400 miles (1,391,000 kilometers).
• This is about 108 times of the Earth. (Diameter: 12,742 km)

Planets Density:

• Earth has the highest density. at 5.514 g/cm3
• Saturn is the least dense.
• Density less than water.
• Which technically means that Saturn would float.
• It is made mostly of hydrogen.

3. Stars

• A star is an astronomical object consisting of a luminous spheroid of plasma held together by its own gravity.
• The nearest star to Earth is the Sun. Distance from the Earth: 8 light minutes.
• The next nearest star is Alpha Centauri. Distance from the Earth: 4.3 light years.
• The most prominent stars were grouped into constellations and asterisms.
• The observable Universe contains an estimated 1×10²⁴ stars. However, most are invisible to the naked eye from Earth.
• None of the stars outside our galaxy, the Milky Way, are visible.
• The stars appear to move from east to west. Stars appears to rise in the east and set in the west.
• Reason: The Earth rotates from west to east on its axis.
• At equator one can see stars rising perpendicular to the horizon.

3.1 Difference Between Planets and Stars

3.2 Important Stars

T Tauri stars (TTS)

• It is a class of variable stars associated with youth (< 10 Myr).
• A variable star is a star whose brightness as seen from Earth fluctuates.

Herbig Ae/Be star (HAeBe):

• A pre-main-sequence star: a young (<10Myr) star of spectral types A or B.
• Still embedded in gas-dust envelopes.
• A pre-main-sequence star or a PMS star: is a star in the stage when it has not yet reached the main sequence.

The pole Star

• It is situated in the direction of the earth’s axis. It does not appear to move.
• It is not too bright.
• It is not visible from the southern hemisphere.

Pulsars

• Rotating neutron stars that emit regular pulses of radiation.
• Formed from the remnants of massive stars that have undergone supernova explosions or from the collapse of massive stars.

Variable stars

Variable stars exhibit changes in luminosity, either periodically or randomly.

These changes can result from intrinsic or extrinsic factors.

Intrinsic Variability: Pulsating Variable Stars:

• Some stars undergo pulsations during their evolution, leading to variations in radius and luminosity.
• Pulsating variables, like Cepheid and Cepheid-like stars, and long-period variables such as Mira, show these fluctuations.
• The pulsation periods can range from minutes to years, depending on the star's size.

Intrinsic Variability: Eruptive Variable Stars:

• Eruptive variable stars experience sudden increases in luminosity due to events like flares or mass ejections.
• Protostars, Wolf-Rayet stars, flare stars, and giant/supergiant stars fall into this category.

Intrinsic Variability: Cataclysmic or Explosive Variable Stars:

• This group includes novae and supernovae, undergoing dramatic changes in their properties.
• Binary star systems with a nearby white dwarf can lead to certain types of explosive events, like novae and Type 1a supernovae.

Extrinsic Variability: Luminosity Changes from External Factors:

• Extrinsic factors, such as eclipsing binaries and stars with extreme starspots, can cause variations in luminosity.
• Algol, an example of an eclipsing binary, regularly varies in magnitude over a specific period.

3.3 Main Sequence Stars

Formation and Maturation:

• A star the size of our Sun requires about 50 million years to mature from the beginning of the collapse to adulthood.
• Sun will stay in this mature phase on the main sequence in the Hertzsprung-Russell Diagram for approximately 10 billion years.

Nuclear Fusion and Energy Outflow:

• Stars are powered by nuclear fusion of hydrogen into helium in their interiors. Energy outflow from the central regions prevents collapse, sustaining the star's structure and brightness.
• Stars spend about 90% of their lifetimes fusing hydrogen into helium in high-temperature-and-pressure reactions in their cores. Such stars are said to be on the main sequence and are called dwarf stars.
• Starting at zero-age main sequence, the proportion of helium in a star's core will steadily increase, the rate of nuclear fusion at the core will slowly increase, as will the star's temperature and luminosity.
• The Sun, for example, is estimated to have increased in luminosity by about 40% since it reached the main sequence 4.6 billion years ago.

Hertzsprung-Russell Diagram:

• Main Sequence stars vary in luminosity and color. Color-magnitude plots are known as Hertzsprung–Russell diagrams.
• Classification is based on these characteristics, spanning from red dwarfs to hypergiants.
• It indicates the physical properties and the progress through several types of star life-cycles.

Red Dwarfs and Hypergiants:

• Red dwarfs, the smallest stars, may have only 10% of the Sun's mass, emitting minimal energy. Despite this, red dwarfs are by far the most numerous stars in the Universe and have lifespans of tens of billions of years.
• Hypergiants, the most massive stars, can be over 100 times the Sun's mass, emitting immense energy but having short lifetimes. Although extreme stars like these are believed to have been common in the early Universe, today they are extremely rare.

Stars and Their Fates:

Life Duration and Size:

• In general, the larger a star, the shorter its life, although all but the most massive stars live for billions of years.
• When a star has fused all the hydrogen in its core, nuclear reactions cease. Deprived of the energy production needed to support it, the core begins to collapse into itself and becomes much hotter.

Red Giant Transformation:

• Hydrogen is still available outside the core, so hydrogen fusion continues in a shell surrounding the core.
• The increasingly hot core also pushes the outer layers of the star outward, causing them to expand and cool, transforming the star into a red giant.

Exotic Nuclear Reactions:

• Sufficiently massive stars support exotic nuclear reactions producing heavier elements. However, such reactions offer only a temporary reprieve as internal nuclear fires become unstable.

Variations and Pulsations:

• Unstable nuclear reactions cause variations in burning intensity.
• Star pulsates and sheds outer layers, creating a cocoon of gas and dust.
• Cepheids are a type of variable star that pulsates in size, causing changes in brightness over time.

Outcome Based on Core Size:

• Core size determines the star's fate after shedding outer layers.
• Various possibilities depending on whether the core is large or small. (Explained below)

3.4 Constellations

• It is a group of stars forms an imaginary outline or pattern, typically representing an animal, mythological object.
• The origins of the earliest constellations likely go back to prehistory.
• The shape of the constellation remains the same.
• The constellation appears to move from east to west.

Ursa Major / Big Dipper / Great Bear / Saptarshi

• There are seven prominent stars in this constellation.
• It appears like a big ladle or a question mark. There are three stars in the handle of the ladle and four in its bowl.

Orion / Hunter

• It can be seen during winter in the late evenings.
• It is one of the most magnificent constellations.
• It has seven or eight bright stars.
• The three middle stars represent the belt of the hunter.
• Arranged in the form of a quadrilateral.
• Sirius star: It is the brightest star. It is located close to Orion.

Cassiopeia

• In the northern sky.
• It is visible during winter in the early part of the night.
• It looks like a distorted letter W or M.

Leo Major Constellation

"Leo" is the Latin word for lion.

Location:

• Positioned between Cancer (the crab) to the west and Virgo (the maiden) to the east.
• Situated in the Northern celestial hemisphere.

Historical Significance:

• Described as one of the 48 constellations by the ancient astronomer Ptolemy.
• Still recognized as one of the 88 modern constellations today.

Distinctive Features:

• Easily recognizable due to its numerous bright stars.
• Possesses a unique and crouching lion-like shape.

The Sickle:

• The lion's mane and shoulders collectively form an asterism known as "The Sickle."
• To contemporary observers, it may resemble a reversed "question mark."

3.5 Asterisms

• It is a popularly known pattern or group of stars that can be seen in the night sky.
• Asterisms do not have officially determined boundaries. It is a visually obvious collection of stars and the lines used to mentally connect them.
• A constellation is an officially recognized area of the sky.

4. Moons in Solar System

Introduction:

• The Solar System contains eight planets and potentially nine dwarf planets.
• Moons, or natural satellites, orbit these celestial bodies.
• There are at least 297 known natural satellites in our Solar System.

Large, Gravitationally Rounded Moons:

• Out of the 297 moons, at least 19 have sufficient mass to be gravitationally rounded.
• Most of these rounded moons have icy crusts. Earth's Moon and Jupiter's Io are notable exceptions with different surface compositions.

Hydrostatic Equilibrium:

• Some of the largest moons maintain hydrostatic equilibrium. They orbit other planets or dwarf planets.
• These moons could be considered dwarf planets or planets if they orbited the Sun directly.

Moon Classification Based on Orbits:

• Moons can be categorized into two groups based on their orbits: regular and irregular moons.
• Regular moons have prograde orbits, following the direction of their planets' rotation, and orbit near their equatorial plane.
• Irregular moons have more diverse orbits, which can be prograde or retrograde, and often have extreme angles relative to their planets' equators.

Origin of Irregular Moons:

• Irregular moons are believed to be minor planets captured from the surrounding space.
• These moons are generally small, with diameters typically less than 10 kilometers (6.2 miles).

Historical Discoveries:

• Galileo Galilei made the earliest recorded discovery of a moon other than Earth's.
• In 1610, he identified the four Galilean moons orbiting Jupiter.
• For centuries, only a few more moons were discovered.
• The Space Age and Moon Discoveries: The 1970s marked a significant increase in moon discoveries due to space missions like Voyager 1 and 2.

5. Astronomical Distance Measurement

1. Astronomical unit (ua or AU)

• One Astronomical unit is roughly the distance from Earth to the Sun.
• 1 AU = about 150 million kilometres (150X10⁶).
• However, the distance varies as Earth orbits the Sun: from a maximum (aphelion) to a minimum (perihelion).

2. Radar Astronomy/ space probes: to obtain precise measurements by means of radar and telemetry.

• It is more accurate: Velocity of radar pulse is accurate to 1 part in 100 million.
• Velocity of light is accurate to 1 part in 1million.

3. Light year:

• It is a unit of length used to express astronomical distances. A light-year is the distance that light travels in vacuum in one Julian year (365.25 days).- International Astronomical Union (IAU).
• 1 Light year = about 9.46 trillion kilometres. Speed of light is about 300,000 km per second.

6. Asteroids

• Asteroids are small, rocky objects that orbit the Sun.
• Asteroids are minor planets, especially of the inner Solar System. Larger asteroids are also called planetoids.
• The sizes varies greatly. Ceres is the largest. Dia: almost 1,000 km. it is massive enough to be a dwarf planet.

• Most of them live in the main asteroid belt—a region between the orbits of Mars and Jupiter.
• Asteroids hang out in other places, too. E.g. some asteroids are found in the orbital path of planets.
• 4 Vesta: It has a relatively reflective surface. Hence, it is the only asteroid which is normally visible to the naked eye.
• Minor Planet Centre: data on almost 8.5 Lakh such objects.

Asteroids as a threat to Earth:

• An asteroid collision is the biggest threat to the planet. (Brief Answers to the Big Questions, Stephen Hawking)
• “It's 100 percent certain we'll be hit by a devastating asteroid, but we're not 100 percent sure when.”- B612 Foundation.
• Space agencies are preparing for a mission to intercept an asteroid.

7. Comets

• A comet is an icy, small Celestial body that, when passing close to the Sun, warms and releases gases by outgassing.
• Outgassing: by the solar radiation (Sun's light pressure) or out-streaming solar wind plasma.
• This produces a visible atmosphere (coma) and sometimes a tail.
• If sufficiently bright, a comet may be seen from Earth without the aid of a telescope.

Structure

• Comets are distinguished from asteroids by the presence of an extended, gravitationally unbound atmosphere surrounding their central nucleus.
• Nuclei:
  • Diameter: few hundred meters to tens of kilometres.
  • Composed of loose collections of ice, dust, and small rocky particles.
• Coma: the central part immediately surrounding the nucleus. Up to 15 times Earth's diameter.
• Tail: a typically linear section consisting of dust or gas blown out from the coma.
  • It may stretch one astronomical unit.
  • The length of the tail grows as it approaches the sun.
  • The tail is always directed away from the sun.

Oort Cloud Region

• Oort cloud region is the main belt for comets. 
• A cloud of predominantly icy planetesimals.
• Trans-Neptunian region.
• Distance from Sun: from 2,000 to 200,000 au. Lies in the in interstellar space.
• The outer limit defines the cosmographical boundary of the Solar System.

Comets vs Asteroids

Orbital period of comets:

• They have a wide range of orbital periods: ranging from several years to potentially several millions of years. Their period of revolution round the Sun is usually very long.
• They revolve around the Sun in highly elliptical orbits.

8. Meteoroids

• A meteoroid is a small rocky or metallic body in outer space.
• Size: Meteoroids are significantly smaller bodies. Range from small grains to one-meter.
  • Objects smaller than this are classified as micrometeoroids or space dust.
• These are fragments from comets or asteroids, or sometimes are collision impact debris.
• Meteoroids are somewhat arbitrarily differentiated from asteroids.
  • The difference between asteroids and meteoroids is mainly of size
  • Meteoroids have a diameter of one meter or less.
  • Asteroids have a diameter of greater than one meter, and as large as 1000 km.
  • Meteoroids can be composed of cometary or asteroidal materials.

8.1 Meteor or shooting star

• When a meteoroid, comet, or asteroid enters Earth's atmosphere, it burns us due to aerodynamic heating.
• These are commonly known as shooting stars, although they are not stars.
• Meteor shower: A series of many meteors appearing to originate from the same fixed point in the sky.

8.2 Meteorite

• It is an object like a meteoroid, comet, or asteroid, which survived its passage through the atmosphere to reach the surface of a planet.
• When the original object enters the atmosphere, it heats up and radiates energy.
  • It then becomes a meteor.
  • Once it settles on the planet’s surface, the meteor becomes a meteorite.
• For geologists, a bolide is a meteorite large enough to create an impact crater.
• An impact event is a collision between astronomical objects causing measurable effects.
• Hoba meteorite in Namibia: the largest known impact meteorite. 60-tonne, 2.7-metre.

• Meteorites help scientists in investigating the nature of the material from which the solar system was formed.

Impact craters:

• Meteoroid collisions with solid Solar System objects create impact craters, which are the dominant geographic features of many of those objects.
• Eg. including the Moon, Mercury, Callisto, Ganymede, and most small moons and asteroids.

9. Minor planet

• An astronomical object in direct orbit around the Sun. It is neither a planet nor exclusively classified as a comet.
• Minor planets can be dwarf planets, asteroids, trojans, centaurs, Kuiper belt objects, and other trans-Neptunian objects.
• The first minor planet to be discovered was Ceres in 1801.

10. Asteroid belt and Kuiper belt

Consists mainly of small bodies or remnants from when the Solar System formed.

11. Trans-Neptunian object (TNO)

• A minor planet in the Solar System that orbits the Sun beyond the orbit of Neptune.
• Eg. Eris (largest), Pluto.

12. Planetesimal

• Solid objects thought to exist in protoplanetary disks and in debris disks.
• They are believed to form out of cosmic dust grains. (Chamberlin–Moulton planetesimal hypothesis)

• The word has its roots in the concept infinitesimal.
• Believed to have formed 3.8 billion years ago in the solar system.
• Valuable in studies of the formation of the solar system.

13. Protoplanet

• A large body of matter in orbit around a star, which is thought to be developing into a planet.
• A large planetary embryo that originated within a protoplanetary disc and has undergone internal melting to produce a differentiated interior.
• These planetary embryos created through the collisions of planetesimals.
• Eg. Vesta is a surviving protoplanet.

14. Protoplanetary disks

• Formed almost immediately after the collapse of a molecular cloud.
• It is a rotating circumstellar disk of dense gas and dust surrounding a young newly formed star. Eg.  a T Tauri star, or Herbig Ae/Be star.
• It is considered as an accretion disk for the star itself.
• In 2018, the first confirmed image of such a disk was reported. It contains a nascent exoplanet PDS 70b.
  • PDS 70 is a low-mass T Tauri star in the constellation Centaurus.

15. Accretion disk

• A circumstellar disk formed by diffuse material in orbital motion around a massive central body.
• Diffused material: gas, plasma, dust, or particles.
• The central body is typically a star.
• Orbiting material spiral inward:
• Loses energy due to friction.
• Loses angular momentum.
• The term accretion refers to the growth in mass of any celestial object due to its gravitational attraction.
• The object whose mass is growing is known as the accretor.
• The formation of stars and planets and the powerful emissions from quasars, radio galaxies etc., all involve accretion disks.

16. Black Hole

• Definition: A black hole is a region of spacetime with immense gravity, preventing anything, even light, from escaping.
• Formation: Predicted by general relativity, formed by a sufficiently compact mass deforming spacetime, leading to the creation of a black hole.
• Event Horizon: The boundary where nothing can escape is called the event horizon, having no locally detectable features per general relativity.
• Black Body: Behaves like an ideal black body, reflecting no light.
• Hawking Radiation: Predicted by quantum field theory, black holes emit Hawking radiation, with temperature inversely proportional to mass, making direct observation nearly impossible.

Historical Development

• 18th Century: John Michell and Pierre-Simon Laplace considered objects with gravitational fields too strong for light to escape.
• 1916: Karl Schwarzschild found the first modern general relativity solution characterizing a black hole.
• 1958: David Finkelstein published the interpretation of "black hole" as an inescapable region of space.
• 1960s: Theoretical work established black holes as a generic prediction of general relativity.
• 1967: Discovery of neutron stars by Jocelyn Bell Burnell fueled interest in gravitationally collapsed objects.
• 1971: Cygnus X-1 was identified as the first known black hole.

Types of Black Holes

• Stellar Mass Black Holes: Form when massive stars collapse at the end of their life cycle.
• Supermassive Black Holes: Can grow by absorbing mass from surroundings, including other stars and black hole mergers. Commonly found at galaxy centers.

Detecting Black Holes

• Interaction with Matter: Black hole presence inferred through interaction with other matter and electromagnetic radiation.
• Accretion Disks: Matter falling onto black holes forms accretion disks, producing bright objects like quasars.
• Tidal Disruption: Stars passing too close to supermassive black holes can be torn apart into streamers, emitting intense radiation.
• Orbiting Stars: Observing stars orbiting a black hole helps determine its mass and location, ruling out alternatives like neutron stars.
• Sagittarius A: At the Milky Way's core, astronomers confirmed the existence of a supermassive black hole, Sagittarius A, with approximately 4.3 million solar masses.