tags:
  - "#astronomy"

Notes on Space

Ecliptic Plane is the plane connecting earth, sun and the earth's orbit around sun. All other sun's planets also live in this plane. This is a default reference plane. Let us call this as earth-sun orbital plane.
Earth's spin axis is tilted 23.5 Degrees wrt ecliptic plane. The plane formed by earth equator is called celestial plane. The angle between ecliptic and celestial planes is 23.5 degrees.
Aphelion is the point of the Earth’s orbit that is farthest away from the Sun. Perihelion is the point of the Earth’s orbit that is nearest to the Sun. The orbit is elliptical but mostly circular only 3% skewed.
The average sun-earth distance (93 million miles) is called 1 AU (Astronmical unit)

In 2023, Note the following:

Position       Distance                 Date
--------------------------------------------------
perihelion    91 million miles          January 4 
aphelion      94 million miles          July 6.

Relative scale: Sun 10cm (grapefruit size) at 35 feet Earth is about pin head size.

Relative distances:

Body        AU

Sun         0.0 
Mercury     0.4
Venus       0.7
Earth       1.0
Mars        1.5
Asteroids   2.8
Jupiter     5.2
Saturn      9.6
Uranus      19.2
Neptune     30.0

Radius of Sun is 432K miles; i.e. Diameter is 864K miles. i.e. The diameter is about 0.01 AU. i.e. Less than 1% of Earth-Sun distance.
Earth diamter is 12.742K Km. i.e. 8K miles. Around 1% of sun diameter!

Apparent diameter of Sun from Earth:

At 1 A.U.(seconds of arc)  1919.  Max: 1952.  Min: 1887.

Throughout the year the angle between earth and Sun could vary by 23 degrees. Similary the star location also may change depending on the season. The true north is not defined by the Sun's position but defined by Earth's fixed tilt. Even though earth's position changes with in eliptical plane by 2 AU, the relative angles of location of all stars remain same except for Sun.
When viewed from Earth, Sun's angle changes every minute; It even changes after every 24 hours. Location angle of Sun of yesterday and today 12:00 Noon is not same.

Why North Pole Star changes over time

The change is caused by a slow “wobble” in Earth's rotation.
Over a period of 26,000 years, our planet's axis draws a big circle on north sky.
So while Polaris will still be around, it won't keep its position as the North Star.
The axis is moving counterclockwise.
This is called Axial precession which is the precession of the equinoxes. (Equinox is the line at which both day and night durations are equal).
The angular radius is about 23.5 degrees.
The precession of earth axis is due to gravitational forces from Sun, Moon, Jupiter, etc.
The earth's orbit also precess over time. It is not same elliptical path but traces out flower-petal shaped path. It is called perihelion procession. (The nearest point from Sun changes every year). This takes about 112,000 years.
Note that the rising star in east (when sun sets in west) also changes with season. The longest visible star in night is towards opposite direction of sun from earth. Since "Opposite direction of Sun" changes as earth revolves around Sun, the location of stars in night changes over the season. On any month, some stars are visible 12 hours in night but some are visible only an hour or so.
In astronomy, the meridian is the great circle passing through the celestial poles, as well as the zenith and nadir of an observer's location.
Culmination point of a celestial body (like star) is the highest point in sky during it's transit when it crosses the local meridian. i.e. If the star's transit path is drawn as Arc from east to west, this is the highest point in that arc. The distance between observer and star probably remains almost same during transit but lower angle of observation means light has to pass through more of atmosphere and lose it's strength.

Apparent Magnitude

Apparent Magnitude is the measure of apparent illuminance.
Vega is a reference magnitude 0. The scale is reverse logarathmic.
The brightest star Sirius is -1 and it is 2.5 times brighter than vega.
Apparent Magnitude (or apparent volume) larger by 1.0, it is smaller by 2.5 times.

Absolute Magnitude

Measure of absolute luminosity
This is also defined to be reverse algorithmic.
It is equal to the apparent magnitude of the object if it were viewed from a distance of exactly 10 parsecs (32.6 light-years).
Sun's absolute magnitude is 4.83
Sirius Absolute magnitude is 1.43
Sirius is truly about 23 times luminous than Sun.
Lower in magnitude of 3.5 means it is 2.5^3.5=23 times brighter.

Top 10 Brightest stars

Following stars are the top 10 apparent brightest stars in that order.

1.Sirius

Other Names:          Rudra (Shiva) Mrgavyadha (Deer Hunter). 
                      Svana, the dog of prince Yudhishtra.
                      Sopdet (Egyptian name) aka Sothis goddess (Greeks)
                      Nile Star.
Constellation:        (Alpha Canis Majoris)
Location:             Lower South
Distance:             8.6 Light Years. (one of the nearest).
Apparent Magnitude:   -1.46V (Sirius A) and 8.44 (Smaller Sirius B) 
                      2.5 times Vega's brightness.
Abs Luminance:        23 times brighter than Sun.
Mass:                 Twice Sun's mass. (2.0M)
Comments:             Binary Star. Sirius A and B. Revolves around every 50 years.

2. Canopus

Other Names:          Agasthia. "Old man of the south pole" for chinese.
Constellation:        Alpha Carinae
Location:             Lower South near Sirius, lower horizon than sirius.
Distance:             310 Light Years.
Apparent Magnitude:   -0.74V . About twice brigher than Vega ?
Abs Luminance:        10,000 times brighter than Sun.
Mass:                 8 times as massive.
Comments:             Much bigger than Sirius and absolutely brighter.

3. Rigil Kentaurus (Alpha Centauri)

Alpha Centauri (α Centauri, Alpha Cen, or α Cen) is a triple star system in the southern constellation of Centaurus. It consists of 3 stars:

Rigil Kentaurus (Alpha Centauri A),
Toliman (B) and
Proxima Centauri (C). Proxima Centauri is also the closest star to the Sun at 4.2465 light-years (1.3020 pc).
Star B revolves around A in approx 80 years cycle. The distance between them varies from 11 to 31 AU.
Aplha Centauri C is located in 13000 AU from AB. i.e. 0.21 light years.
Second binary star to be found after Acrux.

Names:                Rigil Kentaurus, Alpha Centauri, Alpha Centauri A,
                      Binary other star: Toliman, Alpha Centauri B
                      Triple Other star: Proxima Centauri (C).

Mass:                 10% more massive than Sun.
Location:             Lower South east.
Constellation:        Centaurus
Distance:             4.3 light years. (Close to Sun)
Apparent Magnitude:   0.01 (Rigil Kentaurus) + 1.33 (Toliman) together -0.27
                      i.e. Around 10% brighter than Vega.
Abs Luminance:        Together (1.5+0.5), it is twice brighter than Sun.

4. Arcturus

Arcturus is the brightest star in the northern constellation of Boötes.
Apparent visual magnitude of −0.05, it is the third-brightest of individual stars.
Located relatively close at 36.7 light-years from the Sun.
Arcturus is an aging star around 7.1 billion years old that has used up its core hydrogen.
It is about the same mass as the Sun, but has expanded to 25 times its size and is around 170 times as luminous. Its diameter is 35 million kilometres. (Sun diameter is 1.3 million Kms; Earth diameter is 12000 kms)
It culminates during midnight on April 27th. (i.e. Appears at the highest point in sky)

Names: Arcturus, Swati; The Keeper of North. Mass: 8% more massive than Sun. Radius: 25 times Sun's Radius. Location: higher Northern sky. Constellation: Bootes Distance: 36 Light Years. (Not too far, relatively) Apparent Magnitude: Slightly brighter than Vega. Abs Luminance: 170 times Sun. Age: 7.1 Billion years old. Very old.

5. Vega

Vega is the second brightest star in the northern hemisphere (after Arcturus) in the constellation of Lyra.
This star is relatively close at only 25 light-years from the Sun.
Vega has been extensively studied by astronomers, so it is a reference star.
Vega was the northern pole star around 12,000 BCE and will be so again in 13,727, when its declination will be +86° 14′.
Vega is only about a tenth of the age of the Sun, but since it is 2.1 times as massive, its expected lifetime is also one tenth of that of the Sun; Both stars are at present approaching the midpoint of their main sequence lifetimes.
It is rotating rapidly with a velocity of 236 km/s at the equator. This causes the equator to bulge outward due to centrifugal effects.
From Earth, Vega is observed from the direction of one of these poles.
Based on an observed excess emission of infrared radiation, Vega appears to have a circumstellar disk of dust.

Names:                Vega, Abijit
Mass:                 2.1 times more massive than Sun.
Radius:               2.5 times Sun's Radius.
Location:             Northern sky. 38 degrees From Zenith towards North.
Constellation:        Lyra
Distance:             25 Light Years. (Not too far, relatively)
Apparent Magnitude:   1.0. Vega is the reference star for Magnitude.
Abs Luminance:        40 times Sun.
Age:                  455 Million Years Old. (Sun Age: 4.6 Billion)

6. Capella

Capella is the brightest star in the northern constellation of Auriga. It has the Bayer designation α Aurigae, which is Latinised to Alpha Aurigae and abbreviated Alpha Aur or α Aur. Capella is the sixth-brightest star in the night sky, and the third-brightest in the northern celestial hemisphere after Arcturus and Vega. A prominent object in the northern winter sky, it is circumpolar to observers north of 44°N. Its name meaning "little goat" in Latin, Capella depicted the goat Amalthea that suckled Zeus in classical mythology. Capella is relatively close, at 42.9 light-years (13.2 pc) from the Sun. It is one of the brightest X-ray sources in the sky, thought to come primarily from the corona of Capella Aa.

Although it appears to be a single star to the naked eye, Capella is actually a quadruple star system organized in two binary pairs, made up of the stars Capella Aa, Capella Ab, Capella H and Capella L. The primary pair, Capella Aa and Capella Ab, are two bright-yellow giant stars, both of which are around 2.5 times as massive as the Sun. The secondary pair, Capella H and Capella L, are around 10,000 astronomical units (AU)[note 2] from the first and are two faint, small and relatively cool red dwarfs.

Capella Aa and Capella Ab have exhausted their core hydrogen, and cooled and expanded, moving off the main sequence. They are in a very tight circular orbit about 0.74 AU apart, and orbit each other every 104 days. Capella Aa is the cooler and more luminous of the two with spectral class K0III; it is 78.7 ± 4.2 times the Sun's luminosity and 11.98 ± 0.57 times its radius. An aging red clump star, it is fusing helium to carbon and oxygen in its core. Capella Ab is slightly smaller and hotter and of spectral class G1III; it is 72.7 ± 3.6 times as luminous as the Sun and 8.83 ± 0.33 times its radius. It is in the Hertzsprung gap, corresponding to a brief subgiant evolutionary phase as it expands and cools to become a red giant. Several other stars in the same visual field have been catalogued as companions but are physically unrelated.

Names:                Cappella, Alpha Auriga,
                      4 stars system, Aa, Ab, H and L.
                      Brahmma Hrdaya (Heart of Bramma ?)
Mass:                 Aa and Ab about 2.5 times Sun; H and L about half of Sun.
Radius:               Aa and Ab: About 10 times Radius of Sun; H and L about half Sun.
Location:             Northern sky. About 45 Degrees.
Constellation:        Auriga
Distance:             43 Light Years.
Apparent Magnitude:   Slightly less than Vega.
Abs Luminance:        70 times Sun.
Age:                  600 Million Years. (Sun Age: 4.6 Billion)

7. Rigel (Beta Orionis)

Rigel is a blue supergiant star in the constellation of Orion.
Rigel four+ stars system.
Distance is 860 Light years.
Rigel has exhausted its core hydrogen fuel, expanded, and cooled to become a supergiant.

Names:                Rigel, Beta Orionis, 
                      4+ stars system. 

Mass:                 18-24 times Sun.
Radius:               70+ times Sun's radius
Location:             From Zenith, 8 degrees towards south.
Constellation:        Orion
Distance:             860 Light Years (Sun: 4.6 Billion)
Apparent Magnitude:   0.13V (varying) around 25% less brighter than Vega ?
Abs Luminance:        61,500 to 363,000 times as luminous as the Sun.
Age:                  8 Million years. (Sun: 4.6 Billion)

8. Procyon (Alpha Canis Minoris)

A binary star system, Procyon consists of a white-hued main-sequence star A with with a faint white dwarf companion Procyon B. The pair orbit each other with a period of 40.84 years and an eccentricity of 0.4.

Procyon (top left), Betelgeuse (top right) and Sirius (bottom) form the Winter Triangle. Orion is to the right. It is culminating at midnight on 14 January.

Names:                Procyon, Alpha Canis Minoris
                      Binary stars system. Main with White Dwarf companion.
Mass:                 (1.5+0.6) About 2.1 times Sun's Mass.
Radius:               Procyon A twice Sun's Radius. Dwarf is about 1% of Sun Radius.
Location:             +5 Degrees.
Constellation:        Canis Minor
Distance:             11.46 Light Years (Sun: 4.6 Billion)
Apparent Magnitude:   0.34V 
Abs Luminance:        7 times as luminous as the Sun.
Age:                  2 Billion years. (Sun: 4.6 Billion)

Other Stars:

Legends: 
    (Mv)      : Apparent Magnitude (volume)
    Lum       : Real Luminance
    Mass      : Multiplier for Sun's Mass.
    Radius    : Multiplier for Sun's Radius
    Dist      : Distance in Light years
    Ns        : Total number of Stars (2 for binary stars, etc)
    Bayer     : Bayer Designation like Alpha CMI, etc.
    Const     : constellation 
    Decl      : Declination in Degrees


Seq Name       Mv    Lum   Mass Radius Dist        Bayer   Decl Age   Comments
9   Achernar   0.46  3500  6    7      139.ly      α Eri   -57  67Myr Eridanus Const. South 33 deg from horizon
10  Betelgeuse 0.50  126K  16   800    550.ly      α Ori    7   8Myr  Orion. aka Kakshi, Ardra

11 0.61    Hadar   β Cen   390 B1 III

12 0.76 Altair α Aql 17 A7 V 13 0.76 (1.33 + 1.73) Acrux α Cru 320 B0.5 IV, B1 V 14 0.86 (0.75–0.95var) Aldebaran α Tau 65 K5 III 15 0.96 (0.6–1.6var) Antares α Sco 550 M1.5 Iab-Ib, B2.5 V 16 0.97 (0.97–1.04var) Spica α Vir 250 B1 III-IV, B2 V 17 1.14 Pollux β Gem 34 K0 III 18 1.16 Fomalhaut α PsA 25 A3 V 19 1.25 (1.21–1.29var) Deneb α Cyg 2,600 A2 Ia 20 1.25 (1.23–1.31var) Mimosa β Cru 280 B0.5 III, B2 V 21 1.39 Regulus α Leo 79 B8 IVn