19 October 2019 | New Scientist | 51Next in the series
1 Model the equinox
2 Find the North Star
and Southern Cross
3 Test your area’s light
pollution
4 Identify the craters
of the moon
5 Orion and Sirius:
how to star-hop
6 Planet spotting:
Mars, Mercury
and Uranus
When is a star
not a star?
7 Taurus and the zodiacal
constellations
THIS week, we are going to be
star-hopping. We will identify the
most recognisable constellation
visible around now – Orion – and
use it to find Sirius, the brightest
star in the night sky.
You don’t need any equipment,
and as Orion is one of the brightest
constellations, light pollution
shouldn’t be a problem either.
Sirius can be seen in summer in
the southern hemisphere, rising
early in the morning before the
sun, and in the evening when it
sets after the sun. At the moment
in the northern hemisphere,
Sirius will rise above the horizon
at about midnight, and it will
appear earlier through winter.
Orion the Hunter is a great
constellation to start with because
it contains many bright stars. It is
also found on the celestial equator,
which means it can be seen from
anywhere in the world. It appears
in September, and is visible for
most of the night between January
and March.
The best way to find Orion is
to look for the iconic line of three
stars that make the asterism of
Orion’s Belt. Their names are
Alnitak, Alnilam and Mintaka.
The brightest star in Orion and
the sixth-brightest in the night sky
is Rigel, at the bottom right. Rigel
is actually a system of three stars,
the blue supergiant Rigel A with
two fainter companions.
Betelgeuse, at the top left, is
Orion’s shoulder. Although it is
between 11 and 15 times as massive
as the sun, its radius is 950 times
wider. This makes it one of the
biggest stars we know of. It is a redsupergiant, so has used up its core
of hydrogen and is now burning
the helium around its outside.
It is some 8 or 10 million years old,
which is quite old for a supergiant.
Such stars burn their fuel
quickly and brightly. This means
Betelgeuse might explode in a
supernova any day now. However,
before you get excited, “any day
now” in astronomical terms means
any time in the next million years.
Now for the star-hop. Going
from the right side of Orion’s Belt
to the left, carry that line a little
further than the distance between
Betelgeuse and Rigel. At the
end is Sirius, the dog star, in
the constellation Canis Major.Most stars we can see with the
naked eye are binary systems, and
Sirius is no exception. Sirius A and
B orbit each other at a distance of
between 8.2 and 31.5 astronomical
units (1 AU is the distance between
Earth and the sun). Sirius A is
about twice as big and 25 times as
bright as our sun, but Sirius B used
up its hydrogen a long time ago,
became a red giant and is now a
white dwarf. In absolute terms,
that makes Sirius quite a dim star.
The only reason it appears bright
is that it is just 8.6 light years away.
Next week, we will be finding out
how to tell the difference between
a star and a planet, and spotting all
those it is possible to see. ❚The Orion constellation is one of the easiest to spot, and you can
use it to work your way across the sky, says Abigail BeallPuzzles
Quick crossword,
a matchstick puzzle
and a quiz p52Feedback
Strange maths and
tangerine dreams:
the week in weird p53Almost the last word
Shopping green and
fluorescent blue ink:
readers respond p54The Q&A
Anu Ojha reveals
the solar system’s
coolest moons p56Twisteddoodles
for New Scientist
A cartoonist’s take
on the world p53The back pages
ARMAN GOLESTANEHRIGELBETELGEUSEWhat you need
You don't need anything
For next week
Binoculars
Abigail Beall is a science writer
in Leeds, UK. This series is
based on her book The Art of
Urban Astronomy @abbybeall
Stargazing at home Week 5
Hop from star to star
Stargazing at home online
Projects will be posted online each week at
newscientist.com/maker Email: [email protected]ORION'S BELTSIRIUS