In our latest night sky guide we look at the stars of late spring and early summer and ponder just what does the brightness of a star really mean.

image of Bootis-virgo-leo

Looking south on a May evening (Image credit: Colin Johnston, Armagh Planetarium)

We are now well into the merry month of May and the nights are getting much warmer. The beauty and majesty of the stars on a clear dark night will calm the mind and feed the soul. So what better way to pass away a few hours than to take yourself out to a dark spot and cast your eyes upwards to feast on the great selection of bright stars on display at this time of year?

Start with the seven bright stars of the Plough (a faithful stellar companion the whole year round) which is high over our heads at this time of year. Use the handle of the Dipper to ‘arc’ to Arcturus, the brightest star in the night sky at the moment (in the Northern Hemisphere). It is the bottom sparkler in the constellation of Boötes, the herdsman, a pattern which looks like a kite in the sky. From orangey-coloured Arcturus ‘spike to Spica’ which is the brightest star in the faint constellation of Virgo. Virgo, representing the goddess of corn and agriculture, is the second largest of the constellations.  Spica is easy to find because there are no other bright stars in the vicinity.  When you find Spica, you will have located the position of the lady’s hand.  Her head is toward Leo. To the right of Spica also in Virgo there is a very bright celestial object, the only evening planet on view for May, Saturn.

image of Saturn in different bands

Saturnian Triptych: Infrared images of Saturn from the VISIR instrument on ESO’s Very Large Telescope (centre and right) compared with an amateur visible-light image (left) from Trevor Barry (Broken Hill, Australia) obtained on 19 January 2011. The second image is taken at a wavelength that reveals the structures in Saturn’s lower atmosphere, showing the churning storm clouds and the central cooler vortex. The third image is sensitive to emissions from much higher altitudes in Saturn’s normally peaceful stratosphere, but unexpectedly beacons of infrared emission flanking the central cool region over the storm can be seen. (Image credit: ESO/University of Oxford/L. N. Fletcher/T. Barry)

That mighty lion Leo is moving further and further west and will soon be exiting the celestial stage. The bright star Regulus marks the heart of the lion.  Capella in Auriga and Castor and Pollax in Gemini are still clearly visible in the northwest. Capella is the third brightest star currently to be seen in the northern hemisphere after Arcturus and Vega.  As the heavenly twins and the charioteer leave us, the Summer Triangle consisting of the three bright stars Deneb, Vega and Altair, is back in the east and will dominate the sky for the next six months. These celestial jewels guide us to the constellations of Cygnus the Swan, Lyra, the Harp and Aquila the Eagle respectively.

Let’s take a closer look at the stellar triumvirate which maps out the triangle in the sky.

image of Summer Triangle

Deneb (circled), Altair and Vega make up the Summer Triangle (Image credit: Colin Johnston, Armagh Planetarium)

Deneb which marks the tail of the constellation Cygnus the Swan is a bluish-white supergiant star with a diameter about 200 times that of our Sun.  It is one of the brightest stars in the sky and is 1500 light years (460 parsecs) from Earth. Its size and luminosity indicate that it is a dying star and it will go pop or supernova some time in the next few million years.

Vega is the second brightest star in the northern hemisphere, closely rivaling Arcturus.  It is 25 light years (7.7 parsecs) away and is three times the diameter of the Sun. A main sequence star, Vega was the northern pole star around 12,000BC. “Vega” is derived from the words for “swooping eagle” in Arabic.

The most southerly of the three stars of the Triangle, Altair, is the closest to us being only 17 light years (5.2 parsecs) away.  It is also the smallest of the trio at 1.5 times the diameter of our Sun. Altair spins on its axis once every six and a half hours. Astronomers have observed that because of this rapid rotation, it has a peculiar shape, rather like a squashed sphere (think of a smartie). See the animation at the link below for an illustration of this.

Fast spinnng Altair

What is immediately noticeable about a starry sky is the variation in brightness of the stars. There are a few very bright stars that instantly stand out, lots of medium bright stars and heaps of dim stars that are more difficult to see. The brightness of stars is known as their ‘apparent visual magnitude’ often written as ‘magnitude’, ‘visual magnitude’, ‘apparent magnitude’ or just ‘mag’. The magnitude scale is a little strange to astronomical neophytes in that the numbering system is back to front, the brighter the star the lower the number, so a star of magnitude 2 is brighter than one of magnitude 4. Sometimes a plus sign is placed in front of these numbers. Also stars can have a magnitude of zero and the brightest have minus number. This scale also applies to planets so Venus, the brightest object in the sky after the Moon, has a magnitude of -4.4 when at its’ brightest. Saturn, a night sky object throughout May fades from mag +0.5 at the beginning of the month to mag +0.7 by the end of the month. (If you are familiar with the concept of a logarithmic scale this will make perfect sense, otherwise you’ll just have to live with it.)

And indeed the Sun and the Moon are also rated by this scale, at mag -26.74 and -12.74 respectively. Sirius, the brightest star in the celestial sphere, which is not visible at the moment, is a dazzling -1.46 while the North Star (or Polaris) is magnitude 2. Below mag 6 is not visible to the naked eye, unless you have exceptional eyesight and the darkest and clearest of skies. Arcturus, Vega and Capella have magnitudes of -0.04, 0.03 and 0.91 respectively.

image of Hipparcos

The idea of ‘apparent magnitude’ goes all the way back to the Greek astronomer Hipparchus (190BC – 120BC) . (Image credit: via Wikipedia)

Of course we must remember that there are two factors that affect the intrinsic brightness of stars, distance and their size. Therefore there is another scale, the absolute magnitude which takes these variables into account. Visual magnitude is a kind of relative scale. It tells us the brightness of stars comparative to each other as viewed in the night sky from Earth but nothing of the true brightness of a star. Think of it this way, if you were watching a group of scattered people walking on the beach at a distance from you, telling who is the biggest/smallest would be impossible. Round them up and get them to stand in a straight line all at the same distance from you, at about say 5 metres away and it is easy to judge. The same principle applies to detecting the brightness of stars. Absolute magnitude refers to the brightness of stars as if they were all lined up 10 parsecs (32.6 light years) distance from the Earth. Absolute and apparent magnitudes use the same scale. Absolute magnitude of a star is calculated using a formula which takes into account apparent magnitude and how far away the star is, which is easy to find with the aid of modern telescopes and technology. The absolute magnitude of the sun is approximately +4.8, and Sirius is +1.4 which makes Sirius 40 times more luminous than the Sun. Absolute magnitude can tell astronomers a lot about a star composition, size and age as well as its’ brightness. But for the amateur stargazer there is nothing to beat the twinkling of the mass of stars on display on a clear dark night.

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Image of Mary Bulman

Mary Bulman Education Support Officer

(Article by Mary Bulman)


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