General

Mass of celestial bodies like star is calulated from a relation between its luminosity and spectral stuff ??
Distant stars are useful for navigation
- They are so distant that their change in position is not that visible from earth
- Nearby stars which are close enough for their position change to be visible are not used to guide navigation
Degrees of freedom (science): number of parameters that may be changed independently
- Eg: X and Y coordinates in XY-plane
6 degrees of freedom
- Left-right
- Up-down
- Forward-backward
- Roll: angle change sideways
- Yaw: angle change in horizontal plane
- Pitch: angle change in vertical plane
- https://en.wikipedia.org/wiki/Six_degrees_of_freedom
From the earth, planets appear to move from east to west
- Because of earth's rotation
- All stars rise in the east because it's earth that is rotating, not the stars
Position of planets like mars change because unlike moon, its rotation is not mainly focused on the earth
Andromeda galaxy
- Galaxy closest to Milky way
- The only galaxy observable with naked eye
Skymap
Big stars die out faster
Low-mass stars live longer
- Low-mass stars are like 'fossils of universe'
- There are such stars whose life expectancy from now is more than the current age of universe

Sagan quote: "we're all stardust"
Some stars 'eat' other stars
- https://www.space.com/symbiotic-stars-eat-each-other-beyond-milky-way
Mass of sun:
- Observe luminoisty and temperature => mass derivable
- HR-diagram
- https://en.wikipedia.org/wiki/Hertzsprung%E2%80%93Russell_diagram
Original elements = just H and He
- All other elements are result of star explosions??
- High mass elements are formed in star cores (and shell of stars??)
Binary stars
- Eg: Sirus (A and B)
- Two stars which orbit together. Orbits are dependent on each other
First stars in the universe
Sub-atomic particles:
- Neutrino: Thought to be mass-less
- Positron: Particles of anti-matter
  - Electron with positive charge
Gamma rays
- Most energetic in the light spectrum
- Deadly to humans
Categories of stars based on temperature (coolest to hottest): M, K, G, F, A, B, O
- Spectral class from Morgan-Keenan (MK) system
- Sub-categories are numbers ∈ [0,9]
- Corresponds to spectral type ??
Categories of stars based on luminosity: 0 to VII (Roman numerals)
- Luminosity class
Eg: Sun is G2V
- G2: temperature
- V: Luminosity class

Solar system:

Saturn:
- Has rings
- Most number of moons: 146 moons
Jupiter
- Biggest planet
- 95 moons: Including Europa, Io, Callisto
Mercury and Venus have no moons
- Due to sun being more powerful
- https://spaceplace.nasa.gov/how-many-moons/en/

Misc:

Useful apps: SkyMap, stellarium

Constellations

Constellations are groups of stars
Orion:
- 'The hunter'
- Characteristic 'belt': a line of three stars
- Red star to the left of the 'belt': Betelgeuse (Thiruvathira)
- Bright star to the right of the 'belt': Rigel
Gemini
- Near Mars
- Brightest stars: Castor and Pollux (twins)
- Castor, Pollux and Mars form a triangle with the twins as the base

Zodiac constellations

Zodiac constallations are the 12 constellations that lie on the ecliptic plane
Movement of sun with respect to the zodiac was used in the past to calculate time of year
The term zodiac is derived from a Greek word meaning circle of animals.
https://www.constellation-guide.com/constellation-map/zodiac-constellations/

They are:

Zodiac	Animal
Aries	Ram
Taurus	Bull
Gemini
Cancer	Crab
Leo	Lion
Virgo
Libra
Scorpius	Scorpion
Sagittarius
Capricornus	Goat
Aquarius
Pisces	Fish

Cardinal signs: marks the beginning of the four seasons

this was true long ago, but apparently this has shifted a bit

Zodiac	Season
Aries	Spring
Cancer	Summer
Libra	Autumn
Capricorn	Winter

Telescope

Usually kept pointing north
- Allows 5 degrees of freedom this way ??
Spectroscopy
Reflective
Refractive
Space telescope
- Stable environment. No abrupt temperature changes or cloudy skies.
- Eg: Hubble (sent by Discover space shuttle)
- Eg: Chandra (by NASA, not ISRO. Deployed by Columbia space shuttle)
- Eg: Spitzer (deactivated in 2020)
- Eg: Webb

Gravitational force

From Newton's law of universal gravitation:

      m₁m₂
F = G ----
       r²

G is the gravitational constant with an approximate value of 6.67 * 10⁻¹¹ m³.kg⁻¹.s⁻²


+-----+             +-----+
|  m₁ | → F₁   F₂ ← |  m₂ |
+-----+             +-----+

   |         r         |     
   +-------------------+

Sun

Sun is a low mass star.
A yellow dwarf
- https://spaceplace.nasa.gov/all-about-the-sun/en/
A day of sun: 25-36 earth days (depending on the location in sun)
Star that's closest to Earth
Sun's atmosphere = corona
- https://spaceplace.nasa.gov/sun-corona/en/
Solar wind
Solar cycle
- Around 11 years
- Starts and ends at Solar minimum
- Middle part = Solar maximum, when there are most sun spots
- A complete solar cycle => sun's poles switch places
- https://spaceplace.nasa.gov/solar-cycles/en/
Sun spots
- Dark spots
- Dark because they are cooler
- Caused due to strong magnetic fields
Solar flares
- A sudden explosion
- A big enough solar flar can disrupt radio communication on earth
Solar mass = mass of sun
- A standard unit of mass in astronomy

Stellar evolution

Dwarf = relatively stable
Giants = unstable

Massive star

nebula
|
v
|
main sequence
|
v
|
red giant
|
v (explode)
|
supernova ---------------->---------------+
|                                         |
v (remnant ∈ [1.4, 3] solar mass)         v (remnant > 3 solar mass)
|                                         |
neutron star                              black hole
|
v
|
white dwarf
|
v (cools)
|
black dwarf

Some terms

Celestial sphere: a sphere of inifnite radius that emerges from the centre of the earth
Celestial equator: Greate circle formed by extending earth's equator to fill the celestial sphere
Celestial plane:
Ecliptic plane:
Equinox: Points where the celestial and ecliptic equators intersect
- Vernal equinox
- Autumnal equinox
Solistice: When observed from the earth, sun's movement appears to change direction during the solistices
- Summer solistice
- Winter solistice
Declination
Celestial poles: Points on the celestial sphere when north pole and south pole of the earth are extended infinitely along earth's axis of rotation
- Celestial north pole (CNP)
- Celestial south pole (CSP)
Spherical coordinate system ??
Analemma: path traced by a celestial object over the course of an year
- When observed from the same point at the same time every day
Solar analemma:
- Path traced by the sun.
- Shaped like an '8'
Polaris:
- aka Pole star, North star
- Position: <1° from CNP
- Appears as a single eye to naked eye from earth, but actually consists of 3 stars
Apparent magnitude
- Stellar magnitude
- Brightness of stars when observed from the earth
  - Brighter the star, lower the magnitude
- Negative values => brighter
- https://earthsky.org/astronomy-essentials/what-is-stellar-magnitude/
Nebula: a luminescent interstellar medium
- Sometimes forms stars
- Could be gas leftover after the 'death' of a star??

Coordinate systems

Horizontal coordinate system

Coordinates: altitude (a) and azimuth (A) (like lat and long)
Poles: Zenith and nadir
Centre point: point of observation
0° azimuth: north/south (can be chosen)

Sky divided into two hemisphere with the horizon as the separator
Celestial horizon: the great circle separating the hemispheres
Cardinal direction: Usually north
Zenith: Top-most point in the celestial sphere on a vertical line from point-of-observation
- Pole of upper hemisphere
Nadir: Bottom-most point in the celestial sphere on a vertical line from point of observation
- Pole of lower hemisphere
Azimuth: horizontal angle with respect to the point-of-observation
- Measured (in degrees) eastwards from the north
- Either 0° to 360° or -180° to 180°
- North: 0°
- East: 90°
- South: 180°
- West: 270°
Altitude: vertical angle between star and point of observation plane
Disadvantage: Varies with point-of-observation

Equatorial coordinate system

Coordinates: declination (δ) and (right) ascension (h)
Poles: Celestial poles
Centre point: centre of the sun/earth
0°: March equinox

Ecliptic coordinate system

Coordinates: ecliptic latitude (β) and ecliptic longitude (λ)
Poles: Ecliptic poles
Centre point: centre of the sun/earth
0°: March equinox

Galactic coordinate system

Uses approx plane of milky way as its fundamental plane
Centre: Solar system

Calendars

Julian calendar leap years occurred once in every 4 years
- Didn't skip years that are divisible by 100 but not 400
Lunar calendar: Based on the waxing and waning of moons
Solar calendar: Based on ??
Lunisolar calendar: Based on both sun and moon
On an average a day on earth is 365.2422 days
Gregorian calendar day is 365.2425. To compensate leap years are used.

The moon

Moon is earth's only natural satellite.
Earth's gravitational force has got the moon entranced who feels compelled to stay close to earth.
Earth is one of sun's satellite like moon is earth's.
As the earth revolves around the sun, the moon revolves around the earth.
Moon apogee and perigee
Libration: moon appearing changing shape across the lunar cycle
Lunation
Synodic month: 29.53059 days (approx)
Parallax
Farside and darkside

Phases of the moon

Except on new moon and full moon days, the moon would look differently when viewed from the northern and southern hemispheres.

Phase	Northern	Southern
New moon	🌑	🌑
Waxing crescent	🌒	🌘
First quarter	🌓	🌗
Waxing gibbous	🌔	🌖
Full moon	🌕	🌕
Waning gibbous	🌖	🌔
Last quarter	🌗	🌓
Waning crescent	🌘	🌒

During new moon, the moon is sort of located in between the sun and the earth such that only the side that is hidden from earth is illuminated by the sunlight. The part visible to earth is dark.

       +------+ <-     
earth  | moon | <-  sun
       +------+ <-

Likewise during full moon,

+------+ <-     
| moon | <-  sun
+------+ <-            

            earth

Following adapted from fig

		🌓			←
	🌔		🌒		←	☀️
🌕		🌍		🌑	←	Sunlight
	🌖		🌘		←
		🌗			←

At new moon, one side of the moon receives all the sun light, leaving the other side dark. During full moon, the opposite happens and the sunlight falls on the side that is visible from earth.

Phases of moon had a big role to play in days gone by as it was used to figure out the lunar calendar.

Near and far sides

Only side of the moon of the moon is ever visible when we look from earth. This is known as the near-side of the moon.

The other side is the far side. It never becomes visible to us. Though space exploring satellite have successfully photographed it.

neut

Astronomy