Astronomy Flashcards
Explain Earths moon
- The Moon is Earth’s only natural satellite.
- It lies 238,900 miles (3,84,400 km) away from the Earth on average, although the distance varies by about 5 percent during the Moon’s 27.3-day orbit.
- The Moon has one-eightieth of the mass of the Earth. Viewed from Earth, it goes through phases as reflected sunlight makes different portions of it visible.
- Our satellite is thought to have formed about 4.53 billion years ago, when a Mars-size body smashed into the newborn Earth, spewing hot debris out into Earth’s orbit.
The debris subsequently clumped together into the Moon, which gradually cooled.
- Today, it has a layered interior structure, probably with a small, partially fluid core.
- Over time, the Earth’s gravitational pull on the Moon has forced it into “synchronous rotation,” rotating once for every 27.3-day orbit so that one side faces permanently toward Earth.
- The surface is pockmarked with millions of craters, more than five thousand of which are larger than 12 miles (20 km) across. Most formed from the impact of comets and asteroids.
Phases of the moon:
1 New moon 2 Crescent moon 3 First quarter 4 Waxing gibbous 5 Fool moon 6 Waning gibbous 7 Last quarter 8 Decrescent
Explain SUN
- The Sun is the star at the center of our solar system. It lies about 14,96,00,000 km
- The Sun’s composition is almost three-quarters hydrogen, roughly one-quarter helium (by mass), while heavier elements make up less than 2 percent.
- The Sun generates energy by nuclear fusion of hydrogen in its core. Heat moves out to the photosphere, where the sunlight we see originates. Beyond that a thin “corona” expands outward to form the solar wind, a stream of particles that constantly blows out into space.
- Sunspots are temporary, relatively cool patches on the Sun where magnetic fields have suppressed heat transfer to the surface.
- The Sun formed from a collapsing gas cloud about 4.57 billion years ago. Around 5 billion years from now, it will expand into a red giant star, its outer layers engulfing the planets Mercury and Venus, and possibly the Earth. Eventually, it will shrink into a hot and dense white dwarf.
Explain Eclipses
- Eclipses are astronomical events that occur when one body passes in front of another and blocks out its light. The most spectacular ones are total solar eclipses, which occur when the Moon lines up with the Sun, as viewed from Earth. The Moon blocks out the sunlight, briefly turning day into night.
- Chance alignments between Earth, Moon, and Sun create up to two total solar eclipses each year, visible from limited regions of Earth’s surface. Because the Sun and Moon appear the same size in Earth’s skies, the Moon can obscure the Sun for several minutes.
- Partial solar eclipses occur when the Moon blocks only part of the Sun.
- Total lunar eclipses occur when the full Moon moves into the Earth’s shadow and is no longer illuminated by direct sunlight.
The Moon appears dark red due to some sunlight reaching the lunar surface after refracting, or bending, through the Earth’s atmosphere.
- The word eclipse can also refer to the apparent coincidence of more distant bodies—for instance, one star briefly blocking the light of a companion star orbiting around it.
- UMBRA is the region (on earth) of total eclipse and PENUMBRA is the region (on earth) of partial eclipse.
Explain Planets
According to the new definition, an object must meet three criteria in order to be classified as a planet.
- It must orbit the Sun
- It must be big enough to acquire spherical shape
- It must have cleared other objects out of the way in its orbital neighbourhood. To clear an orbit, a planet must be big enough to pull neighbouring objects into the planet itself.
- The solar system has eight planets:
the four rocky terrestrial planets : Mercury, Venus, Earth, and Mars, and
the gas giant planets: Jupiter, Saturn, Uranus, and Neptune.
- They all formed about 4.54 billion years ago, when material clumped together in a disk of gas and dust around the Sun.
Rocky terrestrial planets formed in the warm inner solar system, which favored compounds with high melting points such as metals and silicates.
The giant planets lie beyond the “frost line,” where volatile compounds formed ices that clumped into larger balls capable of capturing heavy atmospheres.
- The orbital distances of the planets are measured in astronomical units (AU), where 1 AU (=14,95,97,871 km) is the Earth–Sun distance.
- A simple numerical relationship called Titius-Bode law predicts the orbit distances. It starts with 0 followed by the doubling-number sequence 3, 6, 12, etc., then adds four to each and divides by ten. The resulting sequence closely matches the planetary orbit distances (with the exception of Neptune), but there’s no physical reason for this—it’s just a coincidence.
- Terrestrial planets Mercury to Mars
Mercury:
is the planet closest to the Sun. It orbits the Sun every eighty-eight days and rotates very slowly so that a Mercury day—the time between one sunrise and the next—is 176 Earth days. Temperatures during the long days can climb to 849°F (450°C) on the planet, which has almost no atmosphere, while night chills the surface to –274°F (–170°C).
Venus:
is the second planet, orbiting the Sun in about 225 days. It’s similar in size to Earth but is often described as the Earth’s “evil twin.”
It has a crushing, heavy atmosphere of carbon dioxide, a greenhouse gas, which bakes the surface to 869°F (465°C), as well as dense clouds of sulfuric acid.
The Earth:
is the Sun’s third planet, followed by
Mars:
which takes 687 days to orbit the Sun. Today, the average temperature on Mars is about –76°F (–60°C) and the atmosphere is thin and dry, but there are large deposits of ice buried below the surface, and ancient surface features such as river beds suggest that it was once warm enough to have water, oceans, and flowing rivers.
- Outer planets Jupiter to Neptune
The four outer planets of the solar system are vast worlds that together contain almost 99 percent of all the matter orbiting the Sun.
The largest is Jupiter:
which is more than eleven times wider than the Earth. Jupiter orbits the Sun every 11.9 years and is famous for colorful banded clouds and the Great Red Spot, a giant storm that has persisted for at least two centuries.
Jupiter has dozens of moons including Ganymede, the largest moon in the solar system.
Saturn:
like Jupiter, is a gas giant mostly composed of hydrogen and helium. It orbits the Sun every 29.5 years and sports the most magnificent example of an orbiting ring system. The rings are packed with ice chunks, some as large as a bus.Beyond Saturn lie,
Uranus and Neptune:
with orbital periods of 84.3 and 164.8 years, respectively. They are often classed as ice giants because they are richer in ices such as water and ammonia than the gas giants. Uranus’s rotation axis has a curiously high tilt, so it effectively rotates “on its side” compared to Earth.
Explain Dwarf planets, Solar system bodies, Asteroids and Comets
- Broadly speaking, a dwarf planet is a medium-size world roughly 1,200 miles (2,000 km) wide that orbits a star.
There are two broadly excepted criteria’s for classifying a celestial body as Dwarf planet.
- It must orbit the Sun
- It must be big enough to acquire a round shape
(While “Solar System Bodies” fulfil only first criteria I.e. It must orbit the Sun)
- They include Pluto, which was classed as a planet until it became clear that there are many similar-size bodies in the outer solar system; the category dwarf planet was introduced in 2006 by IAU (International Astronomical Union) to unite them.
- Asteroids are smaller than dwarf planets. These rocky chunks mainly circle in the “asteroid belt” between Mars and Jupiter, although a few have elongated orbits, some crossing the orbit of Earth.
Astronomers carefully monitor them to find out if they risk striking the Earth in the future, perhaps even causing a mass extinction.
- Comets are big, dusty snowballs that venture toward the Sun from two chilly reservoirs of icy bodies—the “Kuiper belt” beyond Neptune and the more distant “Oort cloud.”
As they approach the Sun and heat up, comets sprout fuzzy atmospheres of gas and dust, and sometimes a long tail.
Explain Heliosphere
- The heliosphere is a vast bubble carved out in space by the solar wind. This bubble envelops all the solar system planets, with its outer boundary marking the region where the solar wind “loses its puff” and interstellar space begins.
- The solar wind blows past all the planets at supersonic speeds of more than 6,21,000 mph (10,00,000 km/h) before slowing down as it encounters resistance from interstellar gas.
The point where solar wind slows below its speed of sound is called the TERMINATION SHOCK.
- Two NASA spacecraft, Voyager 1 and 2, crossed this shock at distances of about 94 and 76 astronomical units (1 AU is the Earth–Sun distance). The shock is probably irregularly shaped and constantly moving.
- Beyond this lies the HELIOPAUSE, the theoretical boundary where the interstellar medium brings the solar wind to a halt.
Voyager 1 is expected to cross the heliopause by 2014. And beyond this is the “BOW SHOCK,” where the interstellar medium hits the outer heliosphere at high speed due to the Sun’s orbital motion around the Milky Way.
Explain how do we measure star distances
- German scientist Friedrich Bessel made the first accurate measurement of a star’s distance from Earth in 1838, using a technique called PARALLAX.
- The apparent position of a nearby star in the night sky will vary slightly at two times, six months apart, because the Earth has shifted by about 186 million miles (300 million km) as it orbits the Sun.
- Bessel measured the angular shift of a star called “61 Cygni” over six months and calculated its distance (about 9.8 light years) by triangulation.
- Modern satellite measurements have allowed astronomers to calculate the distance of more than 1,00,000 stars using the parallax method.
- More distant stars need another tack. Some variable stars act as “standard candles”—the timing of their brightness variations reliably indicates their intrinsic brightness, so the APPARENT BRIGHTNESS reveals the distance.
- Another technique examines the COLOUR OF LIGHT from distant galaxies. The more distant the galaxy, the more its light will be stretched to long wavelengths by the universe’s expansion since the Big Bang.
Explain Stellar Evolution
- Stellar evolution describes the way stars change as they age.
- Stars form when clouds of gas collapse under their own gravitational pull, and the biggest factor in a star’s destiny is its mass.
- The most massive stars live fast and die young, blowing up in supernova explosions after only a few million years, while theoretically the smallest ones can shine for hundreds of billions of years.
- The Sun is a medium-mass star, which will live for about 10 billion years. It is about halfway through its lifespan, spent mostly in a phase called the “main sequence,” during which it generates energy through hydrogen fusion in its core.
- Astronomers chart the main stages of stellar evolution on the Hertzsprung–Russell diagram, which plots the color of a star against its magnitude or luminosity (a measure of its brightness) to reveal patterns.
- Stars can end their lives in different ways. The Sun will end up as an extremely dense white dwarf, a hot ball of matter roughly the size of the Earth that will gradually cool and fade.
Explain Supernova
- A supernova is a brilliant explosion in which a star blows itself to smithereens.
- “Core-collapse supernovae” signal the death of stars more than eight times as massive as the Sun.
- Fusion reactions gradually build up heavy elements in their cores, but when the fuel runs out, there isn’t enough outward pressure to prevent the core suddenly collapsing, sometimes into a black hole. This triggers an outward shock wave that catastrophically blows the star’s atmosphere apart.
- A related phenomenon is the gamma-ray burst, a powerful blast of gamma rays that satellites have detected since the 1960s. Most of these bursts are thought to signal extremely massive, rapidly rotating stars collapsing into black holes.
- “Type Ia” supernovae are another main supernova class. They occur when a small, dense white dwarf star grows more massive, either because a companion star “feeds” it with matter, or because two white dwarfs merge.
When the total mass reaches about 1.38 times the mass of the Sun, the star becomes unstable and collapses with a huge release of energy.
Explain Extrasolar planets
- An extrasolar planet, or exoplanet, is a world that circles a star beyond our Sun.
- More than 500 exoplanets have been discovered in our galaxy since the mid-1990s, suggesting that other planets are common throughout the universe.
- Most of these worlds have been detected by the radial velocity technique. Astronomers use the Doppler effect to test whether a star is wobbling back and forth due to the gravity of an invisible orbiting planet.
Other planet-hunting techniques include the “transit” method—looking for a slight dimming of a star when a dark planet passes in front of it. A handful of exoplanets have been imaged directly.
Many exoplanets found so far are very unlike the planets of the solar system. Some are “hot Jupiters,” giant planets that zoom round their stars in just a few days, others are “super-Earths,” rocky worlds several times as massive as Earth.
Surprisingly, some exoplanets orbit neutron stars. The holy grail is to find potentially habitable planets similar to Earth orbiting “normal” stars like the Sun.
Explain Milky-way
- The Milky Way is the galaxy of stars that hosts our own solar system. It’s a magnificent example of a large spiral galaxy, containing roughly 400 billion stars.
- Most of the Milky Way’s stars lie within a structure shaped like two fried eggs back to back. A vast disk of stars (the egg whites), roughly one hundred thousand light years wide, has a central starry bulge (the yolks) with a supermassive black hole at its center.
The disk has several bright spiral arms where dense gas fuels vigorous star formation. Our own solar system lies in the disk, about twenty-six thousand light years away from the galactic center, which it orbits every 230 million years.
- The Milky Way’s disk is surrounded by a large spherical halo containing old stars and tight-knit balls of stars called globular clusters, while the whole galaxy is embedded in a vast cloud of invisible dark matter.
- Sometimes, the term “Milky Way” is used to mean the dense band of stars that crosses the sky where we look across the plane of the galactic disk.
What are different Galaxy types
- Galaxies are groups of millions or billions of stars bound together by their mutual gravitational pull.
- They also contain interstellar gas and dust, as well as vast quantities of dark matter.
- Galaxies come in three main types.
-Spiral galaxies:
including our home galaxy the Milky Way, have a disk of stars threaded by spiral arms where vigorous star formation takes place.
- Elliptical galaxies:
which include the most massive known galaxies, are spherical or oval in shape. Galaxies that don’t have a spiral or elliptical shape are classified as irregular. - Stardust Galaxies:
Galaxies frequently collide, their gas and dust sometimes combining to trigger vigorous star formation in a new “starburst galaxy.”
- Active galaxies emit enormous amounts of radiation, but most galaxies are dim dwarfs containing less than a few billion stars.
- Galaxies mill around each other in clusters bound by their mutual gravitational attraction, while clusters congregate in “superclusters” spanning several hundred million light-years.
Explain active galaxies
- Active galaxies have bright cores that emit amazingly large amounts of radiation, outshining all their stars. They are so bright that we see them at enormous distances, sometimes so far away that their light has taken more than 13 billion years to travel to Earth.
- They are thought to contain supermassive black holes in their cores, which feed on stars and interstellar gas spiraling toward them in a swirling disk. This matter becomes searingly hot as it swirls inward, so that it emits extremely intense radiation. At the same time, two energetic jets of particles emerge perpendicular to the disk, blasting out across thousands of light-years of space.
- Active galaxies fall into different categories, including QUASARS, SEYFERT GALAXY, and BLAZERS, which have different patterns of light emission. However, astronomers suspect that they’re all similar objects viewed from different angles. For instance, blazars are probably the subset of active galaxies that have one of their jets pointed directly toward the Earth.
Explain Black holes
- Black holes are dark voids in space from which nothing—not even light—can escape.
- A black hole can form when a very massive star dies in a supernova explosion, leaving behind a dense core so heavy that it can’t support its own weight. It collapses to a tiny point of enormous density with an immense gravitational pull.
- Black holes have a theoretical boundary around them called the EVENT HORIZON, which marks the point of no return. The size of a static black hole’s event horizon is proportional to its mass.
The dark, inescapable region of a ten-solar-mass black hole would be roughly 37 miles (60 km) wide.
- Much heavier black holes lurk at the centers of large galaxies. They have masses thousands to billions of times higher than the Sun, but it’s unclear how they formed. Possibly, many smaller black holes merged.
- No one can see black holes because they don’t emit light, but astronomers can sense their presence by watching their gravitational influence on nearby stars and detecting the radiation from infalling gas and dust.
Explain Neutron stars and Pulsars
- Neutron stars are extremely dense collapsed stars sometimes left behind after a core-collapse supernova explosion.
- If the collapsing core is 1.4 to 3 times as massive as the Sun, it will form a neutron star, while a heavier core will collapse into a black hole.
- Neutron stars form because they are massive enough for their own gravity to compress normal matter into a superdense soup of neutrons surrounded by a solid crust of iron nuclei.
Typically, they are about 9 miles (15 km) wide and spin very rapidly, sometimes once every few milliseconds.
- A teaspoon of material from the core of a neutron star would have a mass of roughly a billion tons.
- Neutron stars also have very intense magnetic fields, which accelerate particles into narrow polar beams emitting bright radiation.
- Neutron stars called pulsars are easiest to detect because of their orientation. They happen to be aligned so that they sweep their bright radiation beams across Earth as they spin, so telescopes pick up regular pulses emerging from them.
