The Winter Night Sky

Stars and constellations

Dr Adrian Jannetta MIMIA CMath FRAS

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THE WINTER NIGHT SKY

Summer observing in Northumberland can begin from Augustonwards. Astronomical darkness (when the Sun is more than 18◦

below the horizon) begins from the first week of August.

The Winter Hexagon

Along the Milky Way: from Auriga to Canis Major

Deepsky objects for binoculars and telescopes

Why are there more bright stars at this time of year?

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CONSTELLATIONS

Traditional Romanian Arabic

Humans are pattern seekers! We can see patterns and shapes among the random distribution ofnearby bright stars. Many cultures around the world have had their own traditions for naming thestar patterns they see; many are tied in with history and mythology.

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PTOLEMY’S 48 CONSTELLATIONS

Claudius Ptolemy (c.AD 90 — AD 168) wasGreco-Egyptian writer known for his works inastronomy, astrology, mathematics andgeography.

Ptolemy’s major astronomical contribution washis Magna Syntaxis (the Great Treatise) which isbetter known by its later Arabic title: Almagest.

In the Almagest Ptolemy summarisesastronomical and mathematical knowledge ofhis day. He discusses the motion of the Sun,moon and planets and argues for anEarth-centred universe (geocentric model).

Within the pages of the Almagest are acatalogue of 1022 stars grouped within 48constellations. These patterns form the basis ofthe 88 modern constellations.

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WINTER HEXAGONThe Winter Hexagon is an asterism of brightstars. Popular astronomy magazines andwebsites have been responsible for makingit more well known to amateurs in recentyears!

Capella (α Aur, mag +0.1) is multiplestar system ~42 ly.

Castor (α Gem, mag +1.6) is a binarystar ~52 ly.

Pollux (β Gem, mag +1.3) is anorange giant star ~34 ly.

Procyon (α CMi, mag +0.3) is ayellowish main sequence star ~11 ly.

Sirius (α CMa, mag -1.5) is ~9 ly.Brightest star in the sky.

Rigel (β Ori, mag +0.1) is a bluesupergiant ~800 ly.

Aldebaran (α Tau, mag +0.9) is anorange giant star ~68 ly.

The entire hexagon is visible in the eveningduring the late winter and early spring.

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DEEPSKY OBJECTS: AN INTRODUCTIONNebulas are clouds of gas and dust. Emission nebulaeshine because of energetic processes inside them (e.g.star formation). Reflection nebulae shine because theyilluminated by nearby stars. Dark nebulae are silhouettedagainst a brighter background of stars.

Open clusters are loosely gravitationally bound systemsstars. They formed from the same nebula and maycontain tens, hundreds or thousands of stars. Openclusters have formed throughout the life of the Galaxy.

Globular clusters are strongly bound systems stars.They formed from the same nebula and may containhundreds of thousands or millions of stars. Globularclusters formed at the same time as the Galaxy.

Galaxies are the most distant objects that amateurastronomers can view with telescopes. They are islandsof stars (containing many hundreds of billions of them issome cases). They come in many shapes and sizes.

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AURIGA, THE CHARIOTEER

Auriga represents Erechthonius of Athens, sonof Vulcan, who was born deformed and raisedby the goddess Athena.

Erichthonius was generally credited to be theinventor of the quadriga, the four-horse chariot,which he used in the battle against the usurperAmphictyon, the event that made Erichthoniusthe king of Athens.

His chariot was created in the image of theSun’s chariot, the reason Zeus placed him in theheavens. The Athenian hero then dedicatedhimself to Athena and soon after, Zeus raisedhim into the night sky in honor of his ingenuityand heroic deeds.

The Milky Way passes through Auriga and theconstellation is home to several bright starclusters and bright photographic nebulae.

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AURIGA STAR MAP

Capella (α Aur) is a binary star comprised of twoyellow giant stars. Capella is about 43 light-yearsaway.

Menkalinan (β Auri) is a binary star systemconsisting of a pair of white subgiant stars at adistance of 80 LY.

ǫ Aur is an unusual eclipsing binary system.Epsilon usually shines at 3rd magnitude butundergoes eclipses every 27 years. The eclipseslast about two years. The companion is thoughtto be a dark disk of material surrounding anorbiting pair of stars. The most recent eclipseoccurred in 2009-11.

M 36 is an impressive open cluster containingabout 60 stars visible with binoculars.

M 37 is a stunning open cluster containing about500 stars scattered over an area as large as the fullmoon.

M 38 is an open cluster whose brightest membersseem to form the shape of a cross. Easy target forbinoculars and small telescopes.

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OBJECTS IN AURIGA

ǫ (EPSILON) AURIGAE

Epsilon Aurigae is a yellow supergiant approximately 2000 LY away. It is an eclipsingbinary and the brightness of the star falls by one magnitude every 27 years, witheclipses lasting about 2 years. The nature of the companion has long been a mystery!Some astronomers speculated it might be a blackhole.

An observing campaign in 2009 — 2011 revealed that the eclipses are caused by adusty cloud of material passing in front of the star.

A pair of hot, blue stars are likely embedded in the dusty disk. Further observationsare needed to clarify the situation. Just waiting for the next eclipse...

Artistic rendition of ǫ Aurigae and the opaque dust cloud causingeclipses every 27 years.

TAURUS, THE BULL

According to Greek mythology, the Bull is formassumed by Zeus when abducted Europa, alegendary Phoenician princess.

Only the front portion of the Bull is depicted,supposedly because Zeus/Taurus was partiallysubmerged in water as he made his escape!

The Milky Way passes near Taurus and hasseveral notable star clusters. The brightest andmost recognisable are the Pleiades (also calledSeven Sisters). The head of the Bull is aV-shaped formation of stars comprisinganother cluster called the Hyades.

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TAURUS STAR MAP

Aldebaran (α Tau) is an orange giant star about68 LY away.

Elnath (β Tau) is the second brightest star inTaurus. Ptolemy considered this star to be sharedwith Auriga and was also known as γ Aurigaeuntil the IAU “transferred” it to Taurus in 1928.

M 1 is a supernova remnant which is also knownas The Crab Nebula. It is visible as a misty patchwith small telescopes.

M 45 also known as The Pleiades, is the mostcelebrated example of an open cluster in thenight sky. About 1000 stars in total at a distanceof 430 light-years.

Mel 25 is more popularly known as the Hyadescluster. It is the nearest open cluster to us at adistance of 153 light-years. Aldebaran is aforeground star and not a member of the cluster.

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OBJECTS IN TAURUS

The Pleaides open cluster (M 45). The cluster is important to astronomers because it is used tocalibrate distances to other star clusters in the galaxy. Astronomers can do this because M 45 isclose enough to have it’s distance accurately determined in several independant ways (includingparallax).

OBJECTS IN TAURUS

The Hyades is a large nearby opencluster. Although Aldebaran appearsto be a member of the cluster, it isactually a foreground star unrelated tothe Hyades.

The Hyades is one of the most studiedstar clusters in astronomy and, like thePleaides, is an important rung on thecosmic distance ladder. The Hyades ismuch older than the Pleaides andmany of the brighter stars haveevolved off the main sequence.

The trajectory of the Hyades clusteracross the sky indicates that it has acommon origin with the Beehivecluster (M 44) in Cancer some 600 - 700million years ago.

THE CRAB NEBULA

The Crab Nebula was discovered in the18th century. It can be seen as a mistypatch of light with telescopes. It is thewreckage of a supernova — an explodingstar — and was recorded by severalcivilisations in the the year 1054.

At the heart of the Crab nebula is a veryenergetic, compact remnant of theoriginal star. It was discovered by radioastronomer Jocelyn Bell-Burnell in 1967and is an example of a neutron star(sometimes called pulsars). The Crabpulsar spins about 30 times per secondand is estimated to be about 30 km indiamater with four times the mass of theSun.

GEMINI, THE TWINS

According to Greek mythology the twins weresons of Leda and were named Castor andPollux.

Castor was the son of Tyndareus, king of Spartaand Leda’s husband. However, Pollux was theson of Zeus (he’d previously seduced Leda).This made Castor mortal and Pollux immortal.When Castor died, Pollux pleaded with Zeus togrant his brother immortality. Zeus agreed andplaced both brothers together among the stars.

The ecliptic reaches its most northerly point inthe constellation Gemini; the Sun is in Geminiat the June solstice.

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GEMINI STAR MAP

Castor (α Gem) is the second brightest star inGemini — marginally dimmer than Pollux.Telescopes show Castor to be a binary star withtwo components of slightly unequal brightness.Each component is also a spectroscopic binary. Afaint red companion star lies some distance fromCastor and is itself another spectroscopic binary.This makes Castor a system of 6 stars! Castor is51 LY away.

Pollux (β Gem) is the brightest star in Gemini. Itis an orange giant star about 34 LY away.

M 35 is a prominent open cluster visible as a hazypatch with binoculars but best seen withtelescopes at low magnification. About 200 starsspread over an area as large as the full moon. M35 is about 2,800 LY away.

NGC 2158 is an open cluster in the same field ofview as M35. It is much more remote; about 9,000LY away.

NGC 2392 is more popularly known as theEskimo Nebula. It is a planetary nebula best seenwith telescopes at high magnification.

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OBJECTS IN GEMINI

NGC 2392, also known as the Eskimo Nebula, is a planetary nebula. The name comes from thedistinctive appearance when viewed telescopically. Planetary nebulae occur when solar mass starsshed their outer envelopes following a helium flash; the exposed core — a white dwarf star — isexposed at the centre of the nebula.

THE MYSTERIOUS GEMINGA

This is a map of the sky at gamma ray wavelengths. Gamma rays are very energeticand are an indicator of matter under extreme conditions. Hot gas, heated by supernovashocks, pervades the disk of the Milky Way. Supernova remnants and pulsars in theMW are visible. Other objects beyond the galaxy such as blazars, AGN are also visible.

Surveys carried out in the 1970s and 1980s picked up a strong source of gamma rayswith no obvious physical counterpart within the borders of Gemini. It was namedGeminga — a transcription of the words gh’è minga "it’s not there" in the Lombarddialect of Milan.

GEMINGA: MYSTERY SOLVED

Nearly 20 years after discovery, astronomers discovered a 0.237 second periodicity insoft x-ray emission.

Geminga is a pulsar (a neutron star) — the remnant of a supernova which explodedabout 300,000 years ago. Measurements suggest Geminga is one of the closest neutronstars to the Sun — just 500 light-years away. It is travelling very quickly through space:about 200 km/s.

ORION, THE HUNTEROrion represents a Hunter placed among thestars by Zeus, but there are several variations ofthe myth surrounding him.

In one story, Orion was the son of Poseidon thesea god. He lost his sight as a punishment fortrying to ravish Merope, daughter of KingOenopion. His sight was later miraculouslyrestored. Orion is linked with the Pleiades starcluster in Taurus. The Pleiades were sevensisters, daughters of Atlas and Pleione. Orionfell in love with the Pleiades and pursued themwith amorous intent.

Orion boasted to Artemis (the goddess ofhunting) that he could kill any animal on Earth.She sent a scorpion, which fatally stung Orionon the foot. Orion and the Scorpion weretactfully placed on opposite sides of the sky!

The Milky Way passes near Orion and thisregion looks towards nearby giant molecularclouds. The bright nebulae that can be seen arethe merely the tip of the iceberg!

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ORION STAR MAPBetelgeuse (α Ori) is a red supergiant stararound 500 LY away. The outer layers areunstable and the brightness varies betweenmagnitude +0.1 and +1.1 over periods of fiveyears or more. Good supernova candidate!

Rigel (β Ori) is a blue supergiant star around 800LY away. Telescopes show a close 7th magnitudecompanion.

σ Ori is a showpiece triple star system visiblewith small telescopes.

M 42/3 is the celebrated Orion Nebula.

M 78 is a diffuse reflection nebula surrounding apair of 10th magnitude stars.

NGC 2069 is also known as the 37 cluster. It’s anopen cluster whose brightest members form thenumerals 3 and 7.

Cr 69 is a large scattering of stars which includesOrion’s Belt. The region is best seen withbinoculars.

Cr 70 is an open cluster marking Orion’s head.The brightest member of the cluster is λ Ori.Long exposure images show a bubble of ionisedhydrogen gas centred on the cluster.

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OBJECTS IN ORION

The Orion Nebula (M 42 and M 43). A tight group of four, hot stars (the Trapezium) is found at theheart of the nebula. Two of the brightest Trapezium stars are responsible for making the entireregion glow. M 42 lies about 1,500 LY away.

OBJECTS IN ORION

The Orion Nebula is an active site of star formation. Some of the stars in the nebula are also in theprocess of forming planetary disks. These proto-planetary disks (“proplyds”) can be seensilhouetted against the bright gas of the nebula. Image credit: Hubble Space Telescope.

OBJECTS IN ORION

NGC 2025 (Flame Nebula) and NGC 2023 (Horsehead Nebula)

OBJECTS IN ORION

CANIS MINOR, THE SMALL DOG

Canis Minor represents the smaller of the twohunting dogs accompanying Orion (theHunter) across the sky.

The Milky Way passes through Canis Minorbut there are no deep sky objects for observersequipped with small telescopes!

The brightest star in the constellation, Procyon,is notable for being relatively nearby — just 11LY — and for having a white dwarf companion.

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CANIS MINOR STAR MAP

Procyon (α CMi) is a yellowish mainsequence star just 11 LY away. Procyon is abinary system with a white-dwarfcompanion star.

Gomeisa (β CMi) is a blue-white stararound 170 LY away. It is a rapidly rotatingshell star which undergoes irregularchanges in brightness.

γ CMi is an easy to resolve double star. Thecomponents have magnitudes +4.4 and +13and are widely separated.

Abell 24 is a faint planetary nebula located

3 12◦

SE of Procyon. It is a faint disk of lightwith a diameter of 6 arcminutes andmagnitude +13.6. The nebula is estimated tobe about 1,700 LY away.

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OBJECTS IN CANIS MINOR

Canis Minor is not renowned for deepsky objects! One the brightest is this planetary nebula calledAbell 24. Shining at 14th magnitude, it is a challenging object for moderate sized telescopes. Veryeasy to find - even without GoTo. (Image credit: David Ratledge).

CANIS MAJOR, THE GREAT DOG

Canis Major represents the larger of the twohunting dogs accompanying Orion (theHunter) across the sky.

The Milky Way passes through Canis Majorand there are several notable clusters in thispart of the sky (and in adjacent constellations).

The brightest star in the sky, Sirius, is theleading star in Canis Major. For ancientEgyptian astronomers the heliacal rising ofSirius marked a point in the calendar when theNile was about to flood.

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CANIS MAJOR STAR MAP

Sirius (α CMa) is a white main sequence star justunder 9 LY away. Sirius has a white-dwarfcompanion star visible with large telescopes.

Mirzam (β CMa) is a blue-white giant stararound 800 LY away.

Adhara (ǫ CMa) is blue-white giant star around700 LY away.

Wezea (δ CMa) is yellow supergiant star around1800 LY away.

M 41 is a large, bright open cluster to the south ofSirius. Easily visible with binoculars and smalltelescopes. About 100 stars spread over an areathe size of the full moon.

NGC 2362 is a distant open cluster centred on the4th magnitude star τ CMa.

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OBJECTS IN CANIS MAJOR

KICKING SIRIUS!

KICKING SIRIUS AGAIN!

SIRIUS A AND B

Sirius is a binary star system; it consists of twocomponents. Sirius A is a white mainsequence star about 26 times more luminousthan the Sun. A white dwarf companion —Sirius B — was discovered in 1862. Sirius Borbits the primary with a period of 50 years ina fairly eccentric orbit.

Sirius B is approximately the same mass as theSun, but only the same size as the Earth. Invisible light, it is difficult to observe in all butlarge amateur telescopes. However, whitedwarf stars shine more brightly in x-rays andSirius A is outshone by the small companionin this Chandra X-Ray image.

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ORBIT OF SIRIUS B

Position of Sirius B relative to Sirius A over the current orbit. At furthest separation atelescope of 12 inch aperture should be enough to resolve Sirius B. The main difficultyof observing Sirius B is the brightness of Sirius A in comparison!

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WHY SO MANY BRIGHT STARS IN THE WINTER SKY?

Many of the brightest stars in the night sky are concentrated in the constellation Orionand surrounding constellations.

Since the end of the 20th century a clearer picture of our location in the Milky Way hasemerged to explain this!

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THE ORION SPUR

Spiral structure in the MilkyWay is messy. The spiral armsare not perfect geometricshapes. Bridges and spurs ofstar formation connect andoverlap the broad spiralstructure.

The Sun is located in the OrionSpur — a feature which is verynearby and contains many hot,young stars. The proximity ofthe Orion Spur gives manymore bright stars in thatdirection.

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