A Spectrum of Stars

During evenings at this time of year it’s possible to see a variety of bright stars, each shining with a distinctive colour of starlight! While all stars have intensely hot cores (fusing atoms together at millions of degrees under high pressure), their surface temperatures are much cooler than that. (I use the term “surface” loosely. The round ball of the sun isn’t solid — the curved edge we see is just the radius where its plasma becomes opaque. Just as some clouds seem to have well defined edges, but we could fly straight through them.) Stars’ surface temperatures vary depending on their mass, age, and composition. And the temperature lends a unique spectral colour to their light that is visible using your unaided eyes.

Here’s a way to think about it. If you apply heat to dark iron, it will change colour as it warms, moving from a dull red, though orange and yellow, brightening to white, and ending with the blinding blue-white glow that welders need eye protection from! Stars shine with the same range of light colours, where cool stars are red and often dim, the hottest stars shine with a bright blue-white light, and the others, like our yellow Sun, fall between the two extremes. (The stars are much hotter than the iron in our example, though.)

Astronomers have created a classification system based upon the colour (or spectrum) of a star’s light, which also indicates its temperature. The spectral classes have been given letter designations. The original naming system of A, B, C, D, etc. had to be adjusted as more discoveries were made about the physics of starlight. The result is the modern set of letters O, B, A, F, G, K, M which can be remembered via mnemonic phrases such as “Oh Be A Fine Girl/Guy, Kiss Me”.

(Above: The HR Diagram is used to classify stars by spectral type )colour and temperature). Stars sitting on the Main Sequence are in the middle of their life-cycles. Many of the most well-known stars are labelled here.)

The O-stars are the hottest, with surface temperatures up to 30,000 K. The M-stars are the coolest, having temperatures of about 2,500 K. As with our welding scenario, O-stars are brighter and M-stars are dimmer, bordering on invisibility to human eyes and telescopes. That’s why the brightest stars in our favorite constellations are commonly blue-white.

Our friendly neighbourhood yellow Sun is in the mid-range, a G-type star with a surface temperature of 5,800 K. Of course, the Sun is so close and so bright — we perceive it as white! (The K refers to Kelvins. A temperature change of one Kelvin is the same as a change of one degree Celsius. But the scales have different starting values — so 0° C is the same temperature as -273 K (which is absolute zero). This difference of 273 degrees hardly matters when we’re talking about thousands of K.)

Here are some stars with distinct colours that you can head outside and look for on the next clear evening. If you have binoculars or a telescope, try unfocusing while looking at the star. Each star’s resulting big fuzzy disk will show its colour better.

(Above: The locations of the coloured stars mentioned in the text are scattered throughout the late winter evening sky, shown for 9 pm in early March)

Betelgeuse is the bright star marking Orion’s eastern (left) shoulder. It’s a “cool” M-class red supergiant star about 600 light-years from Earth. Burning at about 3,500 K, it’s old (about 7 million years old) and is expected to blow up in a supernova explosion any time. In fact, it may already have happened and the light hasn’t arrived yet! Don’t worry — we’re in no danger if it does. Betelgeuse wasn’t always an M-type star. It has temporally become one in old age. True M-type stars comprise perhaps 80% of the stars in our galaxy — dominating the population because they burn slowly and live a long time — 200 billion years or longer!

Aldebaran is the bright orange-tinted eye of Taurus (the Bull), which is located just to the right of Orion. Aldebaran has become a K-star in its old age, swelling to a size many tens of times larger than our sun, even though it has only 1.5 times the sun’s mass. This has dropped its temperature to about 3,900 K. The star is located about 65 light-years from Earth. A star named Epsilon Eridani (the River), which is located about 2.4 fist widths to the lower right of Rigel (Orion’s right foot) is a true K-type star. It isn’t very bright, even though it is only 10 light-years from Earth — right in our backyard! But you can see it in binoculars in early evening. By the way, if you’re outside in late evening, the very bright star rising in the east is yet another elderly K-star, summer’s bright Arcturus.

While you can enjoy a G-star, our Sun, all day, a prominent night-time one sits right overhead in March — the bright yellow star Capella in Auriga (the Charioteer). It’s located high in the west during mid-evening, sitting above and between Orion and W-shaped Cassiopeia (the Queen). Capella is actually four stars in a tight arrangement at a distance of about 42 light-years from Earth, but the brightest two are G-stars each burning at about 5,500 K.

Procyon is the very bright golden coloured F-type star that sits below Gemini, and to the east (left) of Betelgeuse. It’s in the 2-star constellation of Canis Minor (the Little Dog). It’s another neighbour only 11 light-years away! Polaris the North Star is another F-type star.

Speaking of dogs, our friend Sirius the Dog Star marks the head of Canis Major (the Big Dog). Sirius is low in the southern sky this month, just below and to the east (left) of Orion. Sirius is the brightest star in the global night sky, mainly because it is so close to us (at only 9 light-years away) and because it is a hot white 9,900 K A-type star! Pollux’ partner Castor, the higher star in Gemini (the Twins), is another A-star — and so are summertime’s Deneb and Altair.

Rigel, which marks Orion’s western (our right) foot, is a bright blue-white B-type star. Saiph, Orion’s other foot, is another one, but it’s much dimmer. Rigel burns at a whopping 12,100 K and shines extremely brightly despite its nearly 900 light-year distance from Earth! It’s just as bright, but almost twice as far away as Betelgeuse!

O-type stars are at the extreme end of the range. They burn hot and die quickly — in only 10 million years or so, compared to our Sun’s expected lifetime of 10 billion years! Because they are so short-lived, they are also rare. An O-star in our winter night sky, named Naos, sits less than a palm’s width above the southern horizon in the constellation of Puppis (the Stern Deck). While this star burns at a blistering 30,000 K, it’s only a modestly bright because it is about 1,100 light-years away! A dimmer, but more easily found O-star called 15 Monocerotis sits about midway between Procyon and Betelgeuse in the Cone Nebula. It’s visible with unaided eyes, but a bit hard to pick out from the crowd.

Let me know how many of these stars you find, and whether you can discern the colours!

Zodiacal light

For about half an hour after dusk during the two week period between now and the new moon on March 17, look west-southwest for a broad wedge of faint light rising from the horizon and centered on the ecliptic. This is the zodiacal light — reflected sunlight from interplanetary dust particles that are concentrated in the plane of the solar system. Try to observe from a location without light pollution, and don’t confuse the zodiacal light with the brighter Milky Way to the northwest.

The Moon and Planets

The moon spends this week rising late in the night and lingering for a time into the southwestern morning sky. It will be waning, too — reaching its Last Quarter phase on Friday morning and ending the weekend as a beautiful plump crescent in the southeastern pre-dawn sky.

Tonight (Sunday), the moon will land a palm’s width to the left of the bright white star Spica in Virgo (the Maiden). When the waning gibbous moon rises just before midnight on Tuesday evening, it will be positioned 3 finger widths to the upper left of Jupiter. The pair of naked eye objects will cross the sky together during the wee hours, eventually moving to a point low in the southwestern pre-dawn sky on Wednesday morning.

(Above: The waning moon passes through the morning planets this week, starting with Jupiter on March 7, as shown here for 5:45 am local time)

On Friday morning, the moon will take up a position a palm’s width to the upper right of reddish Mars. Look for the pairing in the southeastern sky between about 2 am and dawn local time. Meanwhile, the major asteroid (4) Vesta will be positioned 4.5 degrees (four finger widths) to the left of the moon (and 6.5 degrees above Mars). It’s visible in binoculars if you know where to look. I’ll post a sky chart here.

(Above: A detailed view of the path of the asteroid Vesta (shown slightly enlarged) during Spring of 2018. On March 9, the moon will land 4.5 degrees to the asteroid’s right.)

On Saturday morning, the moon will hop to make a squat triangle above Mars and yellowish Saturn. The next morning, the old crescent moon will sit four finger widths to the left of Saturn. Look for the pair of objects low in the south-eastern sky between 5 and 7 am local time.

(Above: The waning moon lands between Mars and Saturn on Saturday, March 10, as shown here for 5:30 am local time. The planets will remin in place after the moon moves on.)

The planetary trio of Jupiter, Mars, and Saturn will remain in place in the pre-dawn sky while the moon passes through them, and afterwards, too. Technically, Jupiter will become an evening object starting this week — rising a little before midnight tonight, then ever earlier over the next few months. The extremely bright planet will reach its highest elevation (about three fist diameters) above the southern horizon by 5 am local time. It will definitely catch your eye as you leave for school or work.

Dimmer, reddish Mars rises at about 2:30 am local time. Mars is sitting 3 fist diameters to the east (lower left) of Jupiter this week, but it is pulling away from Jupiter and gradually approaching Saturn. The yellowish ringed planet rises about 3:15 am local time, putting it a comfortable height of two fist diameters above the southeastern horizon just before the dawn sky begins to brighten. The teapot-shaped constellation of Sagittarius (the Archer) will sit directly below Saturn all year. This part of the sky will begin to rise in late evening during May.

(Above: The inner planets Mercury and Venus are both making an appearance in the western sky after sunset this week. Swift Mercury is climbing faster than Venus. Mercury will reach the top of its orbit in the coming weeks, and then descend, while Venus will continue to follow the Ecliptic (green line) upwards for months. Sky shown for 7 pm local time on Sunday, March 4, 2018)

Bright Venus continues to climb through the western evening twilight this week. You can look for it low above the western horizon after sunset until it sets at about 7:15 pm local time. Much dimmer Mercury will sit about a finger’s width to the right of Venus tonight, and then ascend to sit several finger widths above Venus on the weekend. The best viewing time for Mercury falls between 6:20 and 7 pm local time. In a telescope, both planets will show nearly full disks because they are on the far side of the sun from Earth.

Astronomy Skylights for this week (from February 25th, 2018) by Chris Vaughan.

Keep looking up to enjoy the sky!