APOD: The Veil Nebula (2010 Sep 16)

[neufer]

Until a few years ago, astronomers placed the Cygnus Loop, and therefore the Veil Nebula, more than 2,500 light years distant. In 1999, astronomers using the Hubble Space Telescope revised that estimate to 40 percent closer, or 1,470 light years. Nevertheless, uncertainties and assumptions used in making that estimate left some researchers unpersuaded. In 2003, a team of researchers used data from the FUSE satellite to measure accurately the distance to a star lying behind the Loop; this value, of 1,860 light-years, places an upper boundary on the distance to the Loop and provides an independent confirmation of the shorter distance scale.

Source: http://www.daviddarling.info/encycloped ... ebula.html

Unless one is planning to visit, I suspect exact distance doesn't matter all that much.

Well...yes it does actually.

We know the angular diameter of the Cygnus Loop is nearly 3° across and
one can measure the expansion rate accurately with Doppler techniques.

But we can't know exactly WHEN it exploded (as observed on earth) without
knowing it's exact physical dimension...which requires knowing its distance.

[icehouse]

Where's the black hole that accompanied the supernova?

Supposedly it is hidden in the blackground.

Really, so its just not visible, theoretically? I thought at least S. Hawking believes that there would be something to see? It would play tricks on the mind/eye, but something wouldn't look right. Everything looks perfectly normal. Or, do we believe there is no black hole?

I'm confused.

[Beyond]

Icehouse, when someone gets off on the wrong foot about something, it is often hard to discover the difference between someone tugging at your pant-leg to get your attention and someone just plain pulling your leg. What prompted you ask your question in the first place?

[neufer]

Where's the black hole that accompanied the supernova?

Supposedly it is hidden in the blackground.

Really, so its just not visible, theoretically?

It is almost invisible, theoretically.

I thought at least S. Hawking believes that there would be something to see?

S. Hawking believes that very small black holes left over from the big bang
(assuming that they exist, at all) might be observed randomly disintegrating.

It would play tricks on the mind/eye, but something wouldn't look right.
Everything looks perfectly normal. Or, do we believe there is no black hole?

Astronomers believe there is a black hole within the Cygnus Loop but unless
it should accidentally pass directly in front of another star one would not see it.

Gravitational microlensing experiments hope to observe a black hole someday:

<<Gravitational microlensing allows the study of objects that emit little or no light. When a distant star or quasar gets sufficiently aligned with a massive compact foreground object, the bending of light due to its gravitational field, as discussed by Einstein in 1915, leads to two distorted unresolved images resulting in an observable magnification. The time-scale of the transient brightening depends on the mass of the foreground object as well as on the relative proper motion between the background 'source' and the foreground 'lens' object.

Since microlensing observations do not rely on radiation received from the lens object, this effect therefore allows astronomers to study massive objects no matter how faint. It is thus an ideal technique to study the galactic population of such faint or dark objects as brown dwarfs, red dwarfs, planets, white dwarfs, neutron stars, black holes, and Massive Compact Halo Objects. Moreover, the microlensing effect is wavelength-independent, allowing to study source objects that emit any kind of electromagnetic radiation.

Microlensing by an isolated object was first detected in 1993. Since then, microlensing has been used to constrain the nature of the dark matter, detect extrasolar planets, study limb darkening in distant stars, constrain the binary star population, and constrain the structure of the Milky Way's disk. Microlensing has also been proposed as a means to find dark objects like brown dwarfs and black holes, study starspots, measure stellar rotation, look for cosmic strings, study globular clusters, and probe quasars, including their accretion disks.

Microlensing is caused by the same physical effect as strong lensing and weak lensing, but it is studied using very different observational techniques. In strong and weak lensing, the mass of the lens is large enough (mass of a galaxy or a galaxy cluster) that the displacement of light by the lens can be resolved with a high resolution telescope such as the Hubble Space Telescope. With microlensing, the lens mass is too low (mass of a planet or a star) for the displacement of light to be observed easily, but the apparent brightening of the source may still be detected. In such a situation, the lens will pass by the source in a reasonable amount of time, seconds to years instead of millions of years. As the alignment changes, the source's apparent brightness changes, and this can be monitored to detect and study the event. Thus, unlike with strong and weak gravitational lenses, a microlensing event is a transient phenomenon.

Unlike with strong and weak lensing, no single observation can establish that microlensing is occurring. Instead the rise and fall of the source brightness must be monitored over time using photometry. This function of brightness versus time is known as a light curve.

Light curve of gravitational microlensing event with its model fitted (red)

---

Simulated strong lensing (black hole going past a background galaxy).

In practice, because the alignment needed is so precise and difficult to predict, microlensing is very rare. Events, therefore, are generally found with surveys, which photometrically monitor tens of millions of potential source stars, every few days for several years. Dense background fields suitable for such surveys are nearby galaxies, such as the Magellanic Clouds and the Andromeda galaxy, and the Milky Way bulge. In each case, the lens population studied comprises the objects between Earth and the source field: for the bulge, the lens population is the Milky Way disk stars, and for external galaxies, the lens population is the Milky Way halo, as well as objects in the other galaxy itself.>>

[Ann]

Where's the black hole that accompanied the supernova?

Supposedly it is hidden in the blackground.

I couldn't help giggling at this, but you deserve a more serious answer, icehouse.

The truth is that, while astronomers believe that black holes are created in supernova explosions, not a single black hole has been found that can be directly linked to a detectable supernova remnant.

But there are supernova remnants that definitely contain the extremely compact remnant of the star that once exploded. The most famous of these remnants is the Crab Nebula:


The Crab Nebula is believed to be the remnant of the supernova that was detected by Chinese and Arab astronomers in 1054 AD.

The Crab Nebula contains a pulsar, a rotating neutron star, which is an extremely compact "burnt-out" stellar core that weighs many times more than the Sun, even though it is compressed to a size that is far smaller than the Earth. The Crab Nebula pulsar is located pretty much in the middle of the Crab Nebula:


The Crab Nebula pulsar.

Another supernova remnant that contains a pulsar is the Vela supernova remnant. See APOD, September 10, 2010:http://apod.nasa.gov/apod/ap100910.html

Also see this thread, where you can find posts describing the position of the Vela pulsar inside the Vela supernova remnant:
http://asterisk.apod.com/vie ... =9&t=21046

But not all supernova remnants seem to contain any sort of compact remnant of the star that actually exploded. Probably the best-observed supernova explosion ever is SN 1987A in the Large Magellanic Cloud. Astronomers managed to identify the star that had exploded:

Click to view full size image

The supernova and the star that exploded, before it exploded.

Astronomers know enough about the star that exploded to say that it should leave either a pulsar or a black hole behind. But despite extensive searches with the best telescopes, no compact remnant has been found in the glowing gas cloud that has been left behind:


No compact remnant of any kind has been found here.

On the other hand, it is known that neutron stars and even pulsars can be found where they can not be associated with supernova remnants. The gaseous shells may have disappeared, leaving the neutron star of pulsar seemingly "naked", or the neutron star or pulsar may have been given a "kick" by the supernova explosion itself if that explosion was asymmetric. That way, the pulsar or neutron star may simply have been "flung away" from the glowing gaseous shells.

Geminga is a famous neutron star that isn't clearly associated with any known supernova remnant. You can read about Geminga here: http://en.wikipedia.org/wiki/Geminga

I would like to quote a bit from that Wikipedia article:

Thus, it is supposed that Geminga is a sort of neutron star: the decaying core of a massive star that exploded as a supernova about 300,000 years ago.
...
The proper motion of Geminga is 178.2 mas/year which corresponds to a projected velocity of 205 kilometers per second.[1] This is very fast for a star, comparable to Barnard's star.

So Geminga is a lonely neutron star that is speeding through space. Many astronomers speculate that the neutron star (or possible black hole) that ought to have been left behind by Supernova 1987A may simply have been "kicked out of the system", flung away from the glowing gaseous remnants, by the explosion itself.

As for speeding neutron stars, please click on the link below to see the speeding binary Vela X-1. Vela X-1 contains one massive blue supergiant and one compact stellar core, the remnant of a supernova explosion. This pair is speeding through space so fast that they create a bow shock in front of them:

http://www.eso.org/public/archives/imag ... o9702a.jpg

So while it is known that supernovae can leave behind neutron stars, and while it is known that neutron stars can speed away from the glowing gaseous remnants that mark the spot of the original supernova, not a single black hole has been definitely linked to an existing glowing gaseous supernova remnant. However, a few good stellar mass black hole candidates exist in our galaxy. The best one may be located in the binary star Cygnus X-1. This is a part of what Wikipedia writes about Cygnus X-1:

Cygnus X-1 (abbreviated Cyg X-1)[11] is a well known galactic X-ray source[12] in the constellation Cygnus. It was discovered in 1964 during a rocket flight and is one of the strongest X-ray sources seen from Earth, producing a peak X-ray flux density of 2.3 × 10−23 Wm−2Hz−1 (2.3 × 103 Jansky).[13][14] Cygnus X-1 was the first X-ray source widely accepted to be a black hole candidate and it remains among the most studied astronomical objects in its class. It is now estimated to have a mass about 8.7 times the mass of the Sun[6] and has been shown to be too compact to be any known kind of normal star or other likely object besides a black hole. If so, the radius of its event horizon is probably about 26 km.[15]

The probable black hole in Cygnus X-1 was most likely created when the massive progenitor star exploded as a supernova.

The thing to remember is that not all supernovae explosions leave black holes behind. Far from it. Also remember that there is a whole other class of supernovae, the Type 1a supernovae that are created when massive white dwarfs get "overloaded" by an additional helping of mass. These very common supernovae never leave behind either neutron stars or black holes.

Ann

[neufer]

Many astronomers speculate that the neutron star (or possible black hole) that ought to have been left behind by Supernova 1987A may simply have been "kicked out of the system", flung away from the glowing gaseous remnants, by the explosion itself.

Oops! You were doing great up until this point, Ann.

The remnant compact Supernova 1987A neutron star (or possible quark star) is certainly traveling at less than 1% the speed of light; therefore, it couldn't have moved more than a quarter of a light year. Remnant compact supernova stars (or black holes) cannot outrace their supernova remnants.

[Ann]

Okay, thanks for clarifying that, Art. Thanks for telling both me and icehouse.

Ann