Astronomical Distances
Astronomers are obsessed with distances. Whether it be the
distance to the Sun, our nearest star or nearest galaxy, we always want to know
how far away things are. But you may have noticed that there are several units
of distance used at different times; this is usually related to how far away the
object in question is. And if some of these distances don’t make your head
hurt, then they should!
Astronomical Units
(AU) – when you read about things within our own Solar System, you will
usually see distances described in AU. So how far is 1 AU? An astronomical unit
is defined as the average distance between the Sun and Earth. This equates to
approximately 149.6 million km (usually rounded up to 150 million km) which is
about 93 million miles. So already the numbers are quite big; it is much easier
to talk about the orbits of our neighbouring planets in terms of AUs rather
than kilometres.
Light Years (ly)
– when we talk about things outside our Solar System, such as distances to
other stars or galaxies, you often see light years used as a unit of
distance. Our nearest star is 4.2 light
years away. So how far away is 1 light year? It refers to the distance that a
photon of light travels in a 1 year period. The speed of light is 300,000km per
second, or 1,080 million km per hour (about 670 million miles per hour) so 1
light year equates to a distance of 9,460,000,000,000 km – so a unit of 1 light
year is far easier to handle! Just for comparison, the speed of light is 173AU
per day, so 1 light year equates to over 63,000AU. Technically our nearest
galaxy is one of the Milky Way’s satellite dwarf galaxies, called the Canis
Major Dwarf. This is 25,000 light years away. However, our nearest spiral
galaxy is the Andromeda Galaxy, which is a whopping 2.5 million light year
away. (Do you want your brain to hurt even more?) That equates to
23,700,000,000,000,000,000 km! And that is our nearest galactic neighbour.
Imagine the numbers when you start talking about the most distant galaxies we’ve
discovered (prepare for your brain to start really
aching now!) At the end of last year, astronomers reported the discovery of the
most distant galaxy ever observed. The galaxy, called “z8_GND_5296”, is 13.1
billion light years away (I won’t write that down in kilometres because it will
be a sea of zeros!). So you can see why astronomers use don’t use kilometres
when measuring such vast distances. However, light years are generally only
used when an article or report is being pitched to members of the public.
Professional astronomers prefer to speak about distances in parsecs.
Parsec (Pc) – The
important thing to know about a parsec is that it equates to about 3.26 light
years, or just over 206,000AU (I’ll explain in a minute why it’s not a nice
round number relating to either of those units). The kind of distances involved
in astronomy means that you are often talking about things in terms of Kpc (1
thousand pc), Mpc (1 million pc) or Gpc (1 billion pc). 1 parsec is defined as “the
distance from the Sun to a hypothetical astronomical object which has a
parallax angle of 1 arcsecond”. So what on earth does that mean? First
of all parallax; close one eye then hold a pencil out in front of you at arm’s
length. Then keeping your arm still, change which eye you are looking through.
It will appear as though the pencil has moved. The same is true if you apply
this to the apparent position of the stars relative to the position of Earth
during its orbit around the Sun. Astronomers can measure how far away an object
is by measuring how far it appears to have moved as Earth moves around the Sun.
We know that the distance between the Sun and Earth is 1AU, so you can use some
simple trigonometry to calculate how far away an object is. So back to our
parsec. The celestial sphere is divided into smaller chunks in the same way
that Earth’s latitude and longitude are. It is divided into degrees (360 in
total), arcminutes, and arcseconds. So think back to that definition: “the distance from the Sun to a
hypothetical astronomical object which has a parallax angle of 1 arcsecond” –
parsec is just an abbreviation of parallax
and arcsecond. The little diagram below should help you to
picture this better. There are no actual objects located at 1 parsec, this is
just the definition of how that unit is calculated. The further away the object
is, the less it will appear to move and so the smaller the parallax angle will
be. And vice versa. Our nearest star, Proxima Centauri is just over 1.3 parsecs
away, whereas Galaxy z8_GND_5296 is just under 1Gpc away. Calculating distances
in parsecs is much simpler than trying to use accurate speed of light
calculations and it is a much more useful unit of measure for very distant
objects; those distances which truly are astronomical!
It's all so very far away!
ReplyDeleteIt sure is!
DeleteThanks For Sharing So what is the Astronomical Distances about Earth and Sun
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