How come there is a white spot on this orange blob?
Betelgeuse captured by ALMA
41–50 of 78 posts
Re: Betelgeuse captured by ALMA
#42How come there is a white spot on this orange blob?
The colors are only there to represent brightness, so the white spot is a brighter patch on the star (brighter because it's hotter).
Re: Betelgeuse captured by ALMA
#43Earlier quoted context omitted.
When we notice it exploding it would have actually exploded 600 years prior.
What is the right way to think about this? If we observe a supernova 600LY away, do we say that event is happening "now" from our frame of reference? Or should we think of it as happening "600 years ago", and the light from the event is only now reaching us? If you think of causality itself moving at the speed of light (which of course it does), and think in light cones rather than referring to a nonexistent universa…
It is happening in our NOW and that's all that matters from a cosmic perspective, since no signal can travel faster than light. In that sense there's only the NOW, even when you're observing your friend a few meters away. We might say that physics itself travels at light speed.
Re: Betelgeuse captured by ALMA
#44That's amazing resolving power, even if it's been done (maybe not in the exact same way) for quite a while now. Hopefully newer telescopes like the James Webb Telescope will be able to resolve even the _planets_ around other stars, which we are already able to do with the biggest of exoplanets today (good example -> [1]). [1] http://phenomena.nationalgeographic.com/files/2014/05/1RSX_J...
JWST, like ground-based telescopes now, will be able to separate the light from massive Jupiter-like exoplanets from the light of their host stars. But it won't be able to see things at a resolution like this image of Betelguse (so no surfaces of planets... that's quite a long time away).
Re: Betelgeuse captured by ALMA
#45A 1.64 billion km diameter sphere 600 light years away will have an apparent width of 2.9 x 10^-7 radians. That's roughly equivalent to looking at an object 250nm wide at arm's length. A red blood cell is approximately 8000nm wide. Crazy resolving power.
How many photons ?
Well, let's do the math.
ALMA consists of 66 antennas, most of which are 12 meters in diameter. That's about 7000 square meters of receiving area.
Betelgeuse is 642 light years away, which is 6x10^18 meters. The area of a sphere with that diameter is about 10^38 square meters. So 10^-34 of the power emitted from Betelgeuse ends up falling on the ALMA array.
According to Wikipedia, the luminosity of Betelgeuse is 90-150 thousand solar luminosity units, which is about 4x10^26 watts. Let's call it 10^31 watts. So the total power received from Betelgeuse by ALMA is about a milliwatt.
But that's the total power, and the ALMA array only receives at 0.32 to 3.6 mm. To figure out what proportion of Betelgeuse's power falls in this range we need to integrate over Betelgeuse's spectrum, both the total spectrum and then this range in order to find the ratio. That part of the calculation is not so easy. But let's see what we can do. Let's assume that Betelgeuse has a blackbody spectrum. Its temperature is 3500K. We can use this handy dandy blackbody spectrum calculator:
http://www.spectralcalc.com/blackbody_calculator/blackbody.p...
When you crunch the numbers it turns out that about 10^-7 of the total power falls in the range 0.32 to 3.6mm. So the total power received by ALMA is about 10^-10 watts.
0.32-3.6mm is in the far infrared. A photon at this wavelength has an energy of about one meV, or about 10^-22 Joules. So 10^-10 watts is about 10^12 photons per second.
I don't know how long the exposure times are, but my guess is that they are measured in hours.
Re: Betelgeuse captured by ALMA
#46> When that happens, the resulting explosion will be visible from Earth, even in broad daylight. How much visibility are we talking about?
IIRC we're talking about it reaching apparent magnitude ~ -12, given or taken a couple magnitudes (or about as bright as the full Moon - Just imagine that much light coming from an infinitely tiny dot instead). BTW with a declination of ~ +7° Betelgeuse is very close to the celestial equator, so the supernova will be visible from anywhere on Earth, save a very small region within 7° from the South Pole.
It's fun to ponder how it will be that bright at every point that is the same distance. Imagine a sphere centered on Betelgeuse with a 600 light year radius with Earth on the surface of this sphere. The fact that enough photons reach my eyeballs to be able to see it at night is difficult to comprehend - the amount of energy needed to sprinkle every square millimeter of a sphere that size is just unimaginable. Now make that 100,000 times brighter during a supernova event -- crazy town. I feel like imagining the energy from a star spread out on a galactic scale helps me understand and appreciate the magnitudes better than any large number of luminosity or watts or photos can.
Re: Betelgeuse captured by ALMA
#47Some pictures of and info on the telescope https://directory.eoportal.org/web/eoportal/satellite-missio...
Re: Betelgeuse captured by ALMA
#48My girlfriend wrote the copy text for this piece while she was working at ESO - good to see some astronomy on HN :)
Also of interest, Cambridge's COAST instrument did this almost 2 decades ago using optical interferometry.
http://www.mrao.cam.ac.uk/outreach/radio-telescopes/coast/co...
Re: Betelgeuse captured by ALMA
#49So if our sun were replaced by Betelgeuse, Betelgeuse would engulf all the inner planets. That's huge. It's hard to fathom a star that big.
Re: Betelgeuse captured by ALMA
#50At the risk of uncovering my ancientness, I remember reading astronomy books as a kid which specified that stars are so far away that they can't appear as anything more than dots of light even when viewed through the largest telescopes. Always makes me wonder what can be achieved in the future, especially since we're probably somewhere on an exponential progress curve. Of course, assuming a lot of optimism about not…
Regarding visible spectrum observations, I've been waiting to see if anyone can come up with a way to develop a consumer-accessible instrument that can sample a high enough resolution, to image all of the moon landing sites.
For as long as I can remember, the same thing has been said about the surface of the moon, which is the primary fuel for hoax narratives.
With all the buzz about high-resolution arrays being cobbled together from current-generation megapixel digital cameras, I'd love to see someone pull this off. It'd be pretty cool to know that for a budget of maybe tens of thousands of dollars, and some software skills, it'd be within the reach of hobbyists to snap some legit photos of the original moon landing artifacts as they exist.