Stellar Firework in a Whirlwind

VLT Image of Supernova in Beautiful Spiral Galaxy NGC 1288

Stars do not like to be alone. Indeed, most stars are members of a binary system, in which two stars circle around each other in an apparently never-ending cosmic ballet. But sometimes, things can go wrong. When the dancing stars are too close to each other, one of them can start devouring its partner. If the vampire star is a white dwarf – a burned-out star that was once like our Sun – this greed can lead to a cosmic catastrophe: the white dwarf explodes as a Type Ia supernova.

In July 2006, ESO’s Very Large Telescope took images of such a stellar firework in the galaxy NGC 1288. The supernova - designated SN 2006dr - was at its peak brightness, shining as bright as the entire galaxy itself, bearing witness to the amount of energy released.

NGC 1288 is a rather spectacular spiral galaxy, seen almost face-on and showing multiple spiral arms pirouetting around the centre. Bearing a strong resemblance to the beautiful spiral galaxy NGC 1232, it is located 200 million light-years away from our home Galaxy, the Milky Way. Two main arms emerge from the central regions and then progressively split into other arms when moving further away. A small bar of stars and gas runs across the centre of the galaxy.

The first images of NGC 1288, obtained during the commissioning period of the FORS instrument on ESO's VLT in 1998, were of such high quality that they have allowed astronomers [1] to carry out a quantitative analysis of the morphology of the galaxy. They found that NGC 1288 is most probably surrounded by a large dark matter halo. The appearance and number of spiral arms are indeed directly related to the amount of dark matter in the galaxy's halo.

The supernova was first spotted by amateur astronomer Berto Monard. On the night of 17 July 2006, Monard used his 30-cm telescope in the suburbs of Pretoria in South Africa and discovered the supernova as an apparent 'new star' close to the centre of NGC 1288, which was then designated SN 2006dr. The supernova reached magnitude 16, that is, it was about 10 000 times fainter than what the unaided eye can see.

Using spectra obtained with the Keck telescope on 26 July 2006, astronomers from the University of California found SN 2006dr to be a Type Ia supernova [2] that expelled material with speeds up to 10 000 km/s.

Notes

[1]: "Morphological structure and colors of NGC 1232 and NGC 1288" by C. Moellenhoff et al., A&A 352, L5 (1999) and "Quantitative interpretation of the morphology of NGC 1288" by B. Fuchs and C, Moellenhoff, A&A 352, L36 (1999)

[2]: Type Ia supernovae are a sub-class of supernovae that were historically classified as not showing the signature of hydrogen in their spectra. They are currently interpreted as the disruption of small, compact stars, called white dwarfs, which acquire matter from a companion star. A white dwarf represents the penultimate stage of a solar-type star. The nuclear reactor in its core has run out of fuel a long time ago and is now inactive. However, at some point the mounting weight of the accumulating material will have increased the pressure inside the white dwarf so much that the nuclear ashes in there will ignite and start burning into even heavier elements. This process very quickly becomes uncontrolled and the entire star is blown to pieces in a dramatic event.
Type Ia supernovae play a very useful role as cosmological distance indicators, allowing astronomers to study the expansion history of our Universe, leading to the conclusion that the Universe is expanding at an accelerating rate.

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Distances in the Universe

Why should we know them?

Because they provide a

• starting point for astrophysics: The distance of any object in the Universe is a decisive parameter for understanding its energy generation mechanisms. An uncertainty of a factor of 2 in the distance means an uncertainty in the power output in error by a factor of 4.
• starting point for cosmology: Distances are necessary in order to determine the structure, evolution and age of the expanding Universe.

What is the "distance ladder"?

This is the procedure to measure progressively larger comic distances. Objects with well-known properties are used to calibrate larger, brighter and more distant objects, which can in turn be used to calibrate other distance indicators that may work for objects that are seen to greater distances. After several such steps we get means to measure cosmological distances.

Cosmic rulers and distance units

The kilometer, while very practical to measure distances on the Earth, is much too small a unit to be used in astronomy. Astronomers use two different distance units (rulers) to measure the universe.

The first one, useful within the solar system, is the Astronomical Unit (AU): this is the mean distance of the Earth from the Sun (original definition)

1 AU = 149 597 870 691 m (149.6 million km) = 499.005 light-seconds

One light-second is the distance traveled by the light in 1 second, or 299 792 km.

Another unit, necessary for much larger stellar and extragalactic distances is the light-year (ly). This is the distance traveled by the light during one year:

1 light-year (ly) = 63 240 AU = 9.450 10¹² km

Astronomers also use a third distance unit, the parsec (pc) - see below

Distances in the solar system

How do we measure the distances?

Historically: After the Kepler's Third Law was discovered, astronomers could determine the relative distances in the solar system. However, to learn the trus size of the solar system, the distance of one planet had to be measured in absolute terms, e.g., in kilometres. They early realised that one possibility to do this was to determine the value of 1 AU by means of observaitons of a Venus Transit.

Today: radar and laser measurements provide distances to major and minor planets with an accuracy of a few meters.

How far away are the planets?

Stars and the Milky Way

How do we measure the distances in the Milky Way?

The Milky Way galaxy in which we live is a very large system. If we could see it from the outside, it would ressemble the spiral galaxy NGC 1232 shown in the photo above.

In order to measure the distances to the nearest stars, astronomers use the method of triangulation.

As a result of the motion of the Earth in its orbit around the Sun during the year, the observed position of a nearby star describes a small eclipse on the celestial sphere. The semi-major axis (the angle on the sky) of this ellipse is called the annual parallax (π). The further the star is away, the smaller is the size of the ellipse and therefore the parallax. By measuring the exact value of its parallax, we may therefore find the distance to a star.

Thanks to observations with many telescopes on the ground, as well with the the Hubble Space Telescope and, above all, with the ESA Hipparcos astrometric satellite, we have now measured the parallaxes of stars up to a distance of about 1000 light-years.

The parsec (pc) is another distance unit used for stars and galaxies which derives directly from the above mentioned parallax. One parsec is the distance, from which the semi-major axis of the Earth's orbit is seen under an angle of 1 arcsec. There are 3600 arcseconds to 1 degree and 360 degrees to a full circle, so 1 arcsec is a very small angle indeed. If we have measured the parallax π, then the distance d = 1/π [expressed in pc]. 1 pc = 3.26 ly.

The nearest star, Proxima Centauri, has the parallax 0.77233 arcsec, corresponding to a distance of 1.2931 pc, or 4.22 ly.

Another method is based on the apparent brightness (as we see it in the sky) and the luminosity (the intrinsic brightness, i.e., as it really is) of a star. The astronomers determine the type of a star (its "spectral type") from the observed spectrum and from this they derive its luminosity. Since the apparent brightness of a star decreases with the second power of the distance, a comparison of the luminosity and observed apparent brightness allows to calculate the distance.

Yet another method to determine distances is based on a particular type of stars - the "Cepheids" - unstable supergiant stars of spectral type F-G which pulsate with periods of 2-40 days. In 1912, an American astronomer, Miss Henrietta Leavitt, by studying many hundreds of such cepheids in the Magellanic Clouds, found that there exists a relation between the period and the apparent brightness. Because all these stars were located in the same small galaxy and therefore were at approximately the same distance, she discovered the existence of the so-called period-luminosity relation for cepheids. The slower the pulsation, the higher the luminosity. This basic astronomical relation was later refined by other astronomers, including Walther Baade (1950).

Therefore, by determining the period of a cepheid star and using Leavitt's period-luminosity relation, we can obtain that star's luminosity. Again, a comparison of the observed apparent brightness and this luminosity then gives the distance. The currently most reliable calibration of the period-luminosity relation is obtained by using some galactic cepheids with known trigonometric parallaxes and those in Magellanic Clouds.

How far away are the stars in our Galaxy?

Extragalactic distances

Astronomers now have several methods of varying reliability to measure distances beyond the Milky Way.

The "Cepheid"-method, when used with large ground based telescopes and the Hubble Space Telescope makes it possible to measure reasonably reliable distances up to about 100 million light-years (Mly).

The light curves of erupting stars ("novae") in other galaxies resemble those of the novae in the Milky Way galaxy. There is a relation between their luminosity and rate with which their light fade after the outburst. By observing carefully this brightness decrease after a novae outburst, it is therefore possible to estimate its luminosity. Comparison of the luminosity and measured apparent brightness gives the distance.

Supernovae (stellar explosion during which a massive star is completely destroyed and its material is blown into surrounding space; a dense object - a neutron star or a black hole may possibly remain behind) are very important distance indicators, thanks to their enormous luminosity. They can therefore be seen over immense distances. It appears that the peak luminosity of the so-called type Ia supernovae is about the same for all explosions. Assuming that this is really so, the observed apparent brightness of such a supernovae allows to calculate its distance. This method works up to about 1000 million light-years.

The most used method for galaxies at even large distances is the so-called Hubble-relation. It consists of measuring the redshift (z) by determining how much the spectral lines in the galaxy's light have been shifted from their normal ("laboratory") wavelengths towards longer wavelengths. In 1929, the American astronomer Edwin Hubble found that the spectra of galaxies show a redshift proportional to their distance. We now know that this effect is caused by the expansion of the Universe.

A galaxy at a distance of 1 million ly has a velocity of about 20 km/s (the value of the "Hubble constant"), while the Virgo cluster of galaxies at a distance of about 60 million ly is receding with a velocity that is 60 times faster, about 1200 km/s.

These are just the most important methods. There are other methods based on the characteristic size or brightness of clouds of ionized hydrogen ("H II regions") or entire galaxies, spiral arms of galaxies or their motions. Even more advanced methods make use of the brightest galaxies in clusters or gravitational lenses.

How far away are the galaxies?

Problems

Each of the mentioned methods to determine distances in the Universe has limitations and uncertainties. Astronomers must be careful to take into account uncertainties caused by a great variety of additional effects, e.g., galaxy lunimosity and chemical composition ("metallicity"). All brightness measurements must be corrected for the effect of "interstellar reddening", that is the absorption of the light by interstellar gas and dust along the line-of-sight to the object that is being studied.

A quick trip through the Universe on a light beam

Assume that we send out a beam of light from Earth. How long will it then take for this light - moving with the highest possible speed according to the special theory of relativity (299 792 km/s) - to reach different objects in the Universe? Look at the table below for the answers.