APOD: Globular Cluster M15 from Hubble (2013 Nov 19)

[Coil_Smoke]

Doh! Some one beat me too it already. I should have read everything first. It is surprising it is so clear with few clouds. Is there a relativistic effect where objects that are accelerating can appear to move faster than light when moving perpendicular to the radial vector?

Is there something that appears to be moving faster than light ?

[Nitpicker]

I can't resist. My God, it's full of stars!

Doh! Some one beat me too it already. I should have read everything first. It is surprising it is so clear with few clouds. Is there a relativistic effect where objects that are accelerating can appear to move faster than light when moving perpendicular to the radial vector?

Two people actually (three if you are not "2001 Odyssey").

M15 is a part of the Milky Way galaxy, but appears ~27° South of the (dusty) Galactic Plane. So, at a distance of 35,000 light-years from us, it must also be a long way from the dusty plane -- in the halo. And this image is also clear of any Earthly cloud, thanks to the orbiting Hubble telescope. As for the question in your last sentence, I don't really understand it, but logically, an object travelling slower than light relative to an observer can never appear to travel faster than light relative to the same observer, because if it were travelling faster than light it would not appear at all.

[Coil_Smoke]

This is beautiful Hubble image. I see a cosmic firework unparallelled by any terrestrial burst.

[alter-ego]

The other interesting thing about M15 is that it is one of four globular clusters in the Milky Way known to contain a planetary nebula. The one in M15 is called Pease 1 and would have been visible if exposures taken through an OIII filter had been included (that is if it lies in the field of view of this particular image).

The nebula is visible in this field. The 2004 APOD (one of the links) shows it more clearly as a nebula, and I've overlapped the old APOD with the new.

... I'm not sure if it is definitely visible in the new one. The details for the new image say it contains no narrowband exposures.

There is no doubt about its visibility in the APOD:
The hover image is a false color representation of Pease 1 from the 2004 APOD (courtesy of messier.seds.org. The original Hubble image was taken with narrow band filters.

Click to view full size image 1 or image 2

Certainly narrow-band filters significantly enhance emission-line features such as nebulas, but they are not required for visibility in most cases. It's not surprising that today's APOD reveals Pease 1.

[alter-ego]

(The hover utility works funny now )

My bad, I just fixed it. I was messing with linked image display last week and neglected to realize my changes would affect the hover CSS.

Thanks geckzilla. I was starting to wonder if I was losing control of my browser.

[Markus Schwarz]

Yet if gravity has been figured out, the physicists have done a remarkably poor job of transferring that knowledge to the public (the interested, educated public).

There are plenty of books that explain gravity to an interested reader. To mention just two of them: "A Brief History of Time" by Stephen Hawking or "Black Holes and Time Warps: Einstein's Outrageous Legac" by Kip Thorne, both distinguished researchers in general relativity. Nowadays, it should not be too hard to find good websites that explain general relativity, or astronomy in general (this side is one of them). Similarly, there are TV shows about this, although the quality strongly varies. At least in German universities, there are free public lectures for interested people. So, what do you want?

As Chris said, some aspects of general relativity are intuitive to understand, and these aspects you find in presentations for the general public. However, to really work with general relativity, you must master the differential geometry of four-dimensional pseudo-riemannian manifolds. It took Einstein about ten years to correctly link his intuition to the mathematics. Nowadays, it still takes a physics/mathematics students a few years of study to reach this point. Maybe in a few dozen years, when teaching methods have evolved even further, general relativity can be taught in high school just as Newtonian gravity is today. Keep in mind that Newtonian gravity and its related mathematics seemed to his contemporaries as bizarre and abstract as general relativity seems to most today.

[Guest]

What would the night sky look like from a planet orbiting one of the center stars?

I was coming here to ask the same question....

There would be dozens of stars brighter than the brightest historical supernova
and perhaps a thousand stars brighter than Venus.

Sounds like it would be hard to sleep at night

[neufer]

What would the night sky look like from a planet orbiting one of the center stars?

There would be dozens of stars brighter than the brightest historical supernova
and perhaps a thousand stars brighter than Venus.

Sounds like it would be hard to sleep at night

Probably not much brighter than a full moon.

[Anthony Barreiro]

What would the night sky look like from a planet orbiting one of the center stars?

There would be dozens of stars brighter than the brightest historical supernova
and perhaps a thousand stars brighter than Venus.

Sounds like it would be hard to sleep at night

Probably not much brighter than a full moon.

The absence of a planet to sleep on would be more disturbing than the amount of ambient light.

[geckzilla]

But the extra light would seem normal to any hypothetical life form on any hypothetical planet in a globular cluster. Curiously, if light levels were high enough, such a planet might not have such a sharp divide between nocturnal and diurnal creatures. Sleep might be very different and much less frequent.

[neufer]

But the extra light would seem normal to any hypothetical life form on any hypothetical planet in a globular cluster. Curiously, if light levels were high enough, such a planet might not have such a sharp divide between nocturnal and diurnal creatures. Sleep might be very different and much less frequent.

Any planets would probably either be:

• 1) so close to their sun as to be tidally locked into a permanent dayside or nightside
  
  2) or "free range" so as have no sun and be in permanent night.

[Chris Peterson]

2) or "free range" so as have no sun and be in permanent night.

That might be better described as permanent twilight from the light of all the nearby stars.

[MarkBour]

Here's a naive question. But, if one has much to learn, one needs to ask, right? I took a piece of the image and downloaded it, then zoomed it in 4x. If you are interested, it is from almost the upper right corner of this APOD image.

A 4x zoomed piece at the upper right

I see one nice bright blue star, and many smaller white and orange ones. I'm curious what the two yellow-green looking objects would be about midway vertically and off to the right in my excerpt. And I'm really curious if someone could tell me about the darker smaller dots as well. Lots of blue pixels that look too small to be stars. Are they just noise, or would someone who works with these images have different guidance as to what to make of those? And then there's one bright red spot that looks almost attached to a white star at the lower right.

[geckzilla]

Cosmic rays, Mark. As in, they aren't actually objects.

[Anthony Barreiro]

Cosmic rays, Mark. As in, they aren't actually objects.

Cosmic rays are objects -- very tiny objects, and very far away from the stars in the globular cluster.

[MarkBour]

Cosmic rays, Mark. As in, they aren't actually objects.

Cosmic rays are objects -- very tiny objects, and very far away from the stars in the globular cluster.

Thanks. So, is this a limiting problem with all deep-space astronomy? What I guessed as "noise", is more accurately called cosmic rays? Is this always a background level in all astronomical imaging? Does it effectively limit the transparency of space, in terms of our ability to investigate ? (That is, even if space is perfectly transparent, and even if we were to put a telescope up next to Hubble that was a thousand times as powerful, or a million times as powerful, still there will be limits on our ability to detect signals because they will be lost in the noise.)

[geckzilla]

Cosmic rays are transient so it is very easy to take several exposures and eliminate them completely. They only limit visibility if there's not enough exposures to eliminate them. They're not like noise at all. I don't think things really get lost in the noise. Things get lost in dust or in blindingly bright nearby objects or because they are moving away from us faster than the speed of light. Bigger telescopes would definitely have an advantage.

[neufer]

[b][color=#0000FF]Gurgen Askaryan (Dec. 14, 1928 – 1997)[/color][/b]

---

Cosmic rays are transient so it is very easy to take several exposures and eliminate them completely. They only limit visibility if there's not enough exposures to eliminate them.
They're not like noise at all. I don't think things really get lost in the noise.

<<Gurgen Askaryan discovered and investigated in details various effects accompanying passage of high energy particles through dense matter (liquids or solids). He showed that hadron-electron-photon showers and even single fast particles may produce sound pulses. Ionization losses are quickly converted into heat, and the small region adjacent to trajectory undergoes quick thermal expansion thus generating sound waves. These results gave a new approach to the study of cosmic rays. Before, investigations of cosmic rays were based on direct interaction of cosmic ray particle with a detector. Askaryan’s results made it possible to detect showers and single particles using sound receivers situated at some distance from the event. Several years ago, the registration of energetic particles and showers with sound detectors in sea water was planned as an important part of global monitoring.>>

[Chris Peterson]

Cosmic rays are transient so it is very easy to take several exposures and eliminate them completely. They only limit visibility if there's not enough exposures to eliminate them.

Exactly. So the ability to eliminate them depends on other decisions, which typically involve compromise. And the S/N is reduced in every part of the image where a cosmic ray has been removed.

They're not like noise at all.

Actually, they do represent an actual noise source, in the true mathematical sense. They are very like sky background noise. Consider some extreme example, where you need a very long exposure to detect a faint object. If the exposure requirement were so long that every pixel the object occupies were saturated by cosmic rays, that would certainly represent the loss of information to noise.

I don't think things really get lost in the noise.

That's exactly what they get lost in. The ultimate limit of detection is determined by the S/N.

Bigger telescopes would definitely have an advantage.

Ignoring the matter of optical resolution, bigger telescopes mean more photons in a given exposure time, which means improved S/N in that same time. In principle, there's no difference between using a larger aperture and using a longer exposure time. In practice, however, there are some noise sources that are a function of time- dark current noise and cosmic rays being two of them.

[geckzilla]

Thanks, Chris. To clarify one thing I said about things not getting lost in noise I just thought that given enough exposure time a space telescope would eventually be able to detect almost anything as opposed to noise hopelessly drowning things out no matter what.

I totally admit to posting that with the hope that you'd fix it up.

[Chris Peterson]

Thanks, Chris. To clarify one thing I said about things not getting lost in noise I just thought that given enough exposure time a space telescope would eventually be able to detect almost anything as opposed to noise hopelessly drowning things out no matter what.

Certainly, eliminating the sky background allows for vastly longer exposures in space than on Earth. But the best cameras still have some dark current noise which will limit the maximum possible exposure (whether broken into sub-exposures or not). And if a pixel is statistically more likely to be hit by a cosmic ray before it is hit by a photon from a dim object, that is obviously a fundamental limitation on sensitivity.

[geckzilla]

I have often imagined that some cleverly-written software could selectively reject areas of obvious cosmic ray hits if the data were somehow constantly streaming instead of delivered all at once. This is probably a manifestation of my lack of real understanding about how CCD's work, though.

[neufer]

<<Airglow (also called nightglow) is the very weak emission of light by a planetary atmosphere. This causes the night sky never to be completely dark, even after the effects of starlight and diffused sunlight from the far side are removed. The airglow phenomenon was first identified in 1868 by Swedish scientist Anders Ångström. The airglow at night may be bright enough to be noticed by an observer and is generally bluish in colour. To an observer on the ground it appears brightest at about 10 degrees above the horizon, because very low down, atmospheric extinction reduces the apparent brightness of the airglow.

Even at the best ground-based observatories, airglow limits the sensitivity of telescopes at visible wavelengths. Partly for this reason, space-based telescopes such as the Hubble Space Telescope can observe much fainter objects than current ground-based telescopes at visible wavelengths. For an 8 m unit Very Large Telescope telescope one needs 40 hours of observing time to detect a V=28 magnitude star through a normal V band filter, while the 2.4 m Hubble only takes 4 hours. Reducing the view field size can make fainter objects more detectable against the airglow; unfortunately, adaptive optics techniques that reduce the diameter of the view field of an Earth-based telescope by an order of magnitude only as yet work in the infrared, where the sky is much brighter.>>

[Chris Peterson]

I have often imagined that some cleverly-written software could selectively reject areas of obvious cosmic ray hits if the data were somehow constantly streaming instead of delivered all at once. This is probably a manifestation of my lack of real understanding about how CCD's work, though.

I can certainly imagine something like that, although not with a CCD. But you could have some hypothetical new type of detector that output a signal each time a photon hit it, containing the time and coordinates of that event. With such a rich dataset, all sorts of interesting processing would be possible. With a CCD (and other current spatial detectors), however, most of the time information is lost. At best, you can determine that a certain number of photons hit a specific pixel between two known times, where the difference in those times is typically large compared to the time between photon strikes.

[Nitpicker]

I have often imagined that some cleverly-written software could selectively reject areas of obvious cosmic ray hits if the data were somehow constantly streaming instead of delivered all at once. This is probably a manifestation of my lack of real understanding about how CCD's work, though.

I can certainly imagine something like that, although not with a CCD. But you could have some hypothetical new type of detector that output a signal each time a photon hit it, containing the time and coordinates of that event. With such a rich dataset, all sorts of interesting processing would be possible. With a CCD (and other current spatial detectors), however, most of the time information is lost. At best, you can determine that a certain number of photons hit a specific pixel between two known times, where the difference in those times is typically large compared to the time between photon strikes.

Intuitively (because I've got nothing else) it sounds like a job for lots of short exposures, with the new zero readout-noise sensors you mentioned a few weeks back.