h2g2 - Constellations: Vela 'the Sail'

I must go down to the seas again,
to the lonely sea and the sky,
And all I ask is a tall ship
and a star to steer her by.
And the wheel's kick and the wind's song
and the white sail's shaking,
And a grey mist on the sea's face,
and a grey dawn breaking.
– Sea Fever by John Masefield

Name:	Vela (Latin: 'sail')
Genitive:	Velorum
Short form:	Vel
Area:	500 sq deg
Co-ordinates¹:	09^h, −50°
Origin:	Modern

Vela, the Sail is a wholly southern constellation lying between Puppis to its west, Centaurus, to the east, Carina to the south and Pyxis and Antlia to its north. Its most prominent stars are of second magnitude and it is seen at its highest in the Southern Hemisphere during their summer months. From the northern latitudes of the British Isles, only the northern tip of the constellation rises above the horizon during the winter months. From vantage points further south it is relatively easily located from the close proximity of Sirius ², the brightest star in the night sky, which lies just a few degrees to its north-west with Puppis in between.

Mythology and History

Vela is part of a one-time giant constellation, Argo Navis, which was one of the 48 constellations listed in Ptolemy's star catalogue the Almagest. It represented the ship the Argo, which was built for the celebrated Jason and his 50 Argonauts of Greek mythology. The tale tells the adventures of the Argonauts and how they undertook a quest to retrieve the golden fleece of Aries the Ram from Colchis. Other stories link Argo Navis with the boat that carried Isis and Osiris in Egyptian myth, while Christian bibliology links it with Noah, his Ark, and the great deluge.

Argo Navis was so large that to make it more manageable, the conventional reference to individual stars was being in 'the sail', 'the keel' or 'the stern'. Argo Navis was the only one of Ptolemy's constellations not to be accepted as a constellation in its own right when the IAU³ delineated the boundaries covering the whole celestial globe. They created four new constellations out of the old one, and named them Puppis the Stern, Carina the Keel⁴, Pyxis the Mariner's Compass and Vela.

Stars of Vela

The four-way split created the anomalous situation with most of the other constellations whereby the brightest star in the constellation is given the Greek letter alpha, the next brightest beta, followed by gamma and so on through the complete Greek alphabet. The split did not take into account the positions of the graded stars, so we now have, for example, alpha and beta in Carina, gamma and delta in Vela and the others spread around those two and Puppis.

The brightest star in Vela is Regor, or gamma Velorum. It is a Wolf-Rayet⁵ star which is highly unstable and very hot. Through a small telescope it can be split from its fifth magnitude companion.

Delta and chi Velorum form two points of the 'False Cross' along with epsilon and tau Carinae of the adjacent constellation Carina. These four together are sometimes mistaken for Crux 'the Southern Cross', which is only a few degrees to the east.

HD 77581 is a supergiant runaway star travelling at an estimated speed of 50 miles per second! Such stars are expelled from their usual orbit by the blast from a supernova, in this case probably from the pulsar that accompanies and orbits around it, Vela X-1. This pulsar is the super-dense remains of a supernova which may have imparted the initial kick and sent HD 77581 on its way. It will continue to 'run away' perhaps until it collides with another star, which, much like a snooker ball impacting another, will disturb the comparatively leisurely stroll of the recipient.

HH 47 is a star in the process of forming. It has jets of plasma reaching over 10,000AU at speeds topping 300km/s. A time-lapse video is available for view at Astronomy Picture of the Day.

Star Table

Star	Designation	Name or Catalogue No	Brightness (^m)	Distance (light years)	Remarks
γ Vel	gamma Velorum	Regor	+1.78	847	Wolf-Rayet
δ Vel	delta Velorum	Koo She	+1.96	79.7	Point of the 'False Cross'
λ Vel	lambda Velorum	Al Sulhail al Wazn	+2.21	577	K4
χ Vel	chi Velorum	Markeb	+4.29	901	Point of the 'False Cross'
HD 77581		unnamed	+6.75	6,000	Runaway binary star with pulsar Vela X-1
HH 47	Herbig-Haro 47	unnamed	N/A	1,500	Protostar

Nebulae

The Gum Nebula is an ancient supernova remnant discovered by astronomer Colin S Gum and named after him. Even by supernova standards this must have been something quite spectacular. Estimated to have occurred about a million years ago, anyone watching in prehistoric times would have been treated to a flaring orb larger and brighter than the Moon. Residue of this supernova has expanded over time to cover a massive angular size of more than 40 degrees, making it the largest nebula of its type that can be seen from Earth.

Consider that the full moon has an angular measurement of half of a degree and this will give an indication of the massive displacement of this nebula. Mainly centred within Vela, the nebula spills over into the adjacent Puppis. Viewed from Earth the front of the nebula is some 450 light years⁶ away, while the rear face is all of 1,500 light years distant. Unfortunately, this means that the whole thing is now so diffuse that it cannot be seen with amateur equipment and requires long time-lapse photographic exposures to be revealed.

The Gum Nebula also contains a further nebula within it, created by a much later exploding star. The Vela Supernova occurred about 11,000 years ago and the aftermath can be seen to be creating an awesome expanding debris cloud as it progresses through space. Parts of the shockwave can be seen as a fine filament giving rise to the common name Pencil Nebula (catalogued as NGC 2736). The remains of the star that created the effect is now a super dense pulsar called the Vela Pulsar. It is a neutron star with a greater mass than our own Sun, compressed into a ten-mile-diameter body, rotating on its own axis ten times a second. The Vela Pulsar can be seen as an X-ray source.

NGC 3132 is a planetary nebula near the constellation's border with Antlia. This is sometimes called The Southern Ring Nebula, but it also has the nickname The Eight Burst Nebula.

Star Clusters

There are several star clusters within Vela. A naked eye open cluster containing about 30 stars, NGC 2391 is the most prominent. It looks like it surrounds omicron Velorum, but this is a line-of-sight effect as the two are not associated. NGC 2547 is another open cluster comprising around 80 stars, located near gamma Velorum. A mystery surrounds the star cluster RCW 38 because astronomers don't know why it is surrounded by a diffuse cloud of X-rays.

Meteor Showers

The Puppid-Velids meteor shower occurs only occasionally between 2 and 6 December. It produces meteors at a rate of up to six per hour, but their radiant⁷ is spread across a large area and encroaches into the neighbouring constellation of Puppis. For that reason this shower is sometimes known as the Alpha Puppids.
The Delta Velids are a weak shower occurring between 22 January and 21 February, with a peak rate of one per hour on 5 February.
The Gamma Velids last for two weeks between 1 and 15 January. Their peak rate occurs on 5 January with a slightly better rate of two per hour.

Extrasolar Planets

One star in Vela, HD 73526, a yellow dwarf like our own Sun, has two extrasolar planetary bodies in orbit around it. HD 73526 b and HD 73526 c are just over twice the mass of our own Jupiter and both are gas giants. The innermost completes its year in 187 days, while the outer orbits in almost exactly twice that period, 377 days.

HD 83443 is an orange dwarf which has a gas giant in an extremely close orbit. Its year lasts just under three Earth days!

HD 75289 b is what astronomers term a 'Cloudy Hot Jupiter', it orbits its yellow dwarf star HD 75289 at a mere 0.05AU⁸. A visual representation of the system can be seen here.

HD 85390 b was one of the planets in the bumper announcement of 32 on 19 October, 2009. It's a gas giant orbiting an orange dwarf star in 781 days.

WASP-19 b is a hot gas giant orbiting its yellow dwarf star WASP-19 in less than an Earth day.

HD 85512 b is a super-Earth which has got astronomers excited due to its position in that system's habitable zone.


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