Common units used in astronomy.

While watching astronomical programs on TV or while reading news or article about any astronomical events, we always come across some of the different units of measurement. We don't use these measurement units in our everyday life since they represent very small or very big numbers which we hardly deal with in our normal day to day life. These units are quite simple to understand as they based on very simple concepts in geometry or physics. These units represents distances, time, brightness, temperature and many other entities in the gamut of astronomy.
So Let's see some of them one by one.

Parsec
The Parsec is another unit to measure the distances in astronomy other than AU and Light year. When astronomers use triangulation to determine distances, then Parsec unit is used to measure distances. Generally a parsec is 3.2616 light years or 30,857,000,000,000 km.

Now let us see, how Parsecs are calculated when Trignometry or Parallax is used to measure Distances.

Calculating Distance in Parsec

Now as shown in this figure the distance from the Earth to the Sun (i.e. 1 AU) is taken as the base of the right triangle, which is formed between three points
1. Earth
2. Sun
3. Object of Measurement (any distant star/ or Galaxy)

And the arcsecond is used as the angle. These angles have very small value, That is why it is measured in arcseconds. One Arcsecond is equal to 1/3,600th of a degree.

Arcsecond is calculated by astronomers in following way.
At some given time of the year, an astronomer can line up a distant star or galaxy with a nearby star of known distance from the Earth. Then, six months later, when the Earth is on the other side of its orbit around the Sun, the astronomer will line up the distant star with the known star. But because of the parallax effect, the distant star will seem to have moved and will be an a different angle from the Earth. By measuring that angle and making some calculations, the astronomer can determine the distance to the far-off star or galaxy.Generally measurements of arcsec angle are made in two parts of the Earth's orbit around the Sun, in order to get as great a distance for the base as possible.

Calculating the value of a parsec






calculating parsec

In the diagram above (not to scale), S represents the Sun, and E the Earth at one point in its orbit. Thus the distance ES is one astronomical unit (AU). The angle SDE is one arcsecond (1/3600 of a degree) so by definition D is a point in space at a distance of one parsec from the Sun. By trigonometry, the distance SD is



One AU = 149,597,870,691 m, so 1 parsec ≈ 3.085 678×10¹⁶ metres ≈ 3.261 564 light-years.

Parsec and Kiloparsec
Distances measured in parsecs include distances between nearby stars, such as those in the same spiral arm or globular cluster. A distance of one thousand parsecs is commonly denoted by the kiloparsec (kpc). Astronomers typically use kiloparsecs to measure distances between parts of a galaxy, or within groups of galaxies. So, for example:

• One parsec is approximately 3.262 light-years.
• The nearest known star to the Earth, other than the Sun, is Proxima Centauri, 1.29 parsecs away.
• The center of the Milky Way is about 8 kpc from the Earth, and the Milky Way is about 30 kpc across.
• The Andromeda Galaxy (M31) is slightly less than 800 kpc away from the Earth.

Even if Parsec Unit is used frequently to measure the distance of distant star and galaxies, its not used that popularly by many scientist or other people.

Common units used in astronomy.

While watching astronomical programs on TV or while reading news or article about any astronomical events, we always come across some of the different units of measurement. We don't use these measurement units in our everyday life since they represent very small or very big numbers which we hardly deal with in our normal day to day life. These units are quite simple to understand as they based on very simple concepts in geometry or physics. These units represents distances, time, brightness, temperature and many other entities in the gamut of astronomy.
So Let's see some of them one by one.

Light year
A light year is simply measured as the distance that light travels in one year.Light year unit is used to represent distances in the Milky Way and distances to closer Galaxies.

Light travels the distance about 186,287.5 miles per each second, so we will get the distance travelled by light within one year in following way.

One light year = Total seconds in year * Speed of light per second
Total seconds in year = Seconds for each day * days in year.

Seconds for each day = Hours in day * Seconds in hour.
................................ = 24 * 60 * 60
................................ = 86,400

Days in each year = 365.25 ( averaged )

Total seconds in year = 365.25 * 86,400
................................ = 31, 557, 600 seconds in a year

So,using this we can calculate one light year as follows,
One light year = 31,775,600 * 186,287.5
..................... = 5,878,786,100,000 miles !

So its almost about 6 Trillion miles.
And its Equals to about 63,241.1 Astronomical Units.

These are the distances in terms of Light year for some of the known objects in sky.

• Earth is 8 light minutes away from Sun.
• Pluto is 5.7 light hours from Sun.
• Our Galaxy The Milky Way is about 100,000 light-years across.
• Closest Galaxy to us i.e. Andromida Galaxy is 2.5 million light-years away.
• Closest start to us, other than Sun is Proxima Centuri and that is 4.2 l years away.

But Universe is so big that for some far distances even higher units like light year becomes inconvinient.


Magnitude.
Magnitude is a unit used to measure the brightness of any celestial object. Its a logarithmic scale.
The apparent magnitude (m) of a celestial body is a measure of its brightness as seen by an observer on Earth, normalized to the value it would have in the absence of the atmosphere. The brighter the object appears, the lower the value of its magnitude. Sun has the highest magnitude (i.e. lower value) and it is –26.73, Hubble telescope can see objects up to magnitudes of 30, which are not only unvisible to naked eye but are really really faint.

Below is the Magnitude of some of the familiar objects in sky
Full Moon has Magnitude of -12.6
The Crab nebula has Magnitude of -6.0
Maximum brightness of Venus has Magnitude -4.6
Maximum brightness of Venus -2.9
Maximum brightness of Jupiter -2.9
Brightest star other than Sun has Magnitude of -1.47
Our closest Galaxy Andromeda Galaxy has Magnitude of 3.4

Common units used in astronomy.

While watching astronomical programs on TV or while reading news or article about any astronomical events, we always come across some of the different units of measurement. We don't use these measurement units in our everyday life since they represent very small or very big numbers which we hardly deal with in our normal day to day life. These units are quite simple to understand as they based on very simple concepts in geometry or physics. These units represents distances, time, brightness, temperature and many other entities in the gamut of astronomy.
So Let's see some of them one by one.

AU or Astronomical Unit
It is the Unit for measurement of astronomical distances.
One Astronomical Unit is approximately the mean distance between the Earth and the Sun.















As you can see in the figure, one Astronomical Unit is a measure of a distance between Sun to the orbit of the earth. Since earth's orbit is not exactly circular, it keeps varying as earth moves through its orbit and thus its distance from the Sun, So the one AU value is a averaged from max and min value of the distance from earth to Sun.
Its value is 149,597,870 km (about 93 million miles).
The AU is most commonly used for the distances of objects with in our solar system. Pluto, the last planet in the solar system is found at an average distance of 39.47 au from the Sun. So its roughly more than 39 times away than that of a distance between earth and sun.

These are the distances in terms of AU for some of the known objects in sky.

• The Earth is 1.00 AU from the Sun
• Mars is 1.52 AU from the Sun
• Jupiter is 5.20 AU from the Sun
• Proxima Centauri (the nearest star to Earth, excluding our own Sun) is approximately 268 000 AU away from the Sun
• Sun's gravitational field's influence (i.e. Hill/Roche sphere) is 125,000 AU away. This distance is roughly 1.8-2.0 light-years
• And our own Sun is 1.7 × 10⁹ AU away from the center of the Milky Way.