Starship Asterisk*

by **MarkBour** » Fri Aug 04, 2023 4:22 pm

This is one of the most beautiful and dramatic images of a prominence I've ever seen. And it's amazing to me that it was produced with an amateur telescope on the ground. Fine work, Mike Wenz! (If I were in Arizona at any time in the last month, I would not be out trying to photograph the Sun. I'd be hiding in some place with air-conditioning.)

by **VictorBorun** » Wed Aug 02, 2023 10:57 am

Locutus76 wrote: ↑Tue Aug 01, 2023 10:04 pm

Today’s APOD really makes me think of the wonderful artwork of the late Don Lawrence, creator of the Storm comics

or like this:

by **alter-ego** » Wed Aug 02, 2023 12:14 am

Chris Peterson wrote: ↑Tue Aug 01, 2023 10:47 pm
alter-ego wrote: ↑Tue Aug 01, 2023 10:38 pm
Chris Peterson wrote: ↑Tue Aug 01, 2023 9:19 pm

But photons don't slow down. They only have one speed... c.
Actually they do. Strictly speaking, "c" specifies the velocity only within a vacuum. The speed of light within a material having index of refraction n is the speed of light in a vacuum ÷ n (c/n). Also, Cherenkov radiation occurs when a charged particle travels faster than light in a medium. The published standard velocity for c is always within vacuum. Measured within our atmosphere (STP conditions), will yield a velocity that is 0.02899% slower the value of "c".
Photons only travel in a vacuum. Once they encounter matter, they scatter, which (in terms of QM) is a process of absorption and re-emission, and that's why we see the "speed of light" as less than c outside of a vacuum. The photons themselves can only travel at c, like any massless particle.

Yeah, the devil's in the details for the source of apparent photon slowdown. Given an atom's quantized orbitals, how do you explain a continuum of reemitted frequencies / energies? I.e, this absorption-reemission process in media works for all frequencies of light. Given our current physics knowledge, Quantum Mechanics has the final say, and I agree the nitty-gritty is in the photon / atom(s) interactions. However, the concept of phase velocity's dependence on the refractive index (c/n) is a useful and practical view on light propagation in media (including Schnell's Law), and the specific time-delay details don't change the observed parameters.
This reminds me of earlier discussions attempting to describe interference in classical and QM formats.

by **Chris Peterson** » Tue Aug 01, 2023 10:47 pm

alter-ego wrote: ↑Tue Aug 01, 2023 10:38 pm
Chris Peterson wrote: ↑Tue Aug 01, 2023 9:19 pm
alter-ego wrote: ↑Tue Aug 01, 2023 8:56 pm
Index of refraction,n, >1 (vacuum = 1), light slows down by c/n, photon energy is constant therefore frequency remains unchanged, so wavelength is shorter.
But photons don't slow down. They only have one speed... c.
Actually they do. Strictly speaking, "c" specifies the velocity only within a vacuum. The speed of light within a material having index of refraction n is the speed of light in a vacuum ÷ n (c/n). Also, Cherenkov radiation occurs when a charged particle travels faster than light in a medium. The published standard velocity for c is always within vacuum. Measured within our atmosphere (STP conditions), will yield a velocity that is 0.02899% slower the value of "c".

Photons only travel in a vacuum. Once they encounter matter, they scatter, which (in terms of QM) is a process of absorption and re-emission, and that's why we see the "speed of light" as less than c outside of a vacuum. The photons themselves can only travel at c, like any massless particle.

by **alter-ego** » Tue Aug 01, 2023 10:38 pm

Chris Peterson wrote: ↑Tue Aug 01, 2023 9:19 pm
alter-ego wrote: ↑Tue Aug 01, 2023 8:56 pm
johnnydeep wrote: ↑Tue Aug 01, 2023 3:24 pm The light of hydrogen link says:

Why is the wavelength slightly shorter in air?
Index of refraction,n, >1 (vacuum = 1), light slows down by c/n, photon energy is constant therefore frequency remains unchanged, so wavelength is shorter.
But photons don't slow down. They only have one speed... c.

Actually they do. Strictly speaking, "c" specifies the velocity only within a vacuum. The speed of light within a material having index of refraction n is the speed of light in a vacuum ÷ n (c/n). Also, Cherenkov radiation occurs when a charged particle travels faster than light in a medium. The published standard velocity for c is always within vacuum. Measured within our atmosphere (STP conditions), will yield a velocity that is 0.02899% slower the value of "c".

by **Locutus76** » Tue Aug 01, 2023 10:04 pm

Today’s APOD really makes me think of the wonderful artwork of the late Don Lawrence, creator of the Storm comics

by **orin stepanek** » Tue Aug 01, 2023 9:44 pm

Beautiful prominence!

Very hot though!

by **Chris Peterson** » Tue Aug 01, 2023 9:19 pm

alter-ego wrote: ↑Tue Aug 01, 2023 8:56 pm
johnnydeep wrote: ↑Tue Aug 01, 2023 3:24 pm The light of hydrogen link says:

H-alpha (Hα) is a deep-red visible spectral line of the hydrogen atom with a wavelength of 656.28 nm in air and 656.46 nm in vacuum. It is the first spectral line in the Balmer series and is emitted when an electron falls from a hydrogen atom's third- to second-lowest energy level.
Why is the wavelength slightly shorter in air?
Index of refraction,n, >1 (vacuum = 1), light slows down by c/n, photon energy is constant therefore frequency remains unchanged, so wavelength is shorter.

But photons don't slow down. They only have one speed... c.

by **johnnydeep** » Tue Aug 01, 2023 9:01 pm

alter-ego wrote: ↑Tue Aug 01, 2023 8:56 pm
johnnydeep wrote: ↑Tue Aug 01, 2023 3:24 pm The light of hydrogen link says:

H-alpha (Hα) is a deep-red visible spectral line of the hydrogen atom with a wavelength of 656.28 nm in air and 656.46 nm in vacuum. It is the first spectral line in the Balmer series and is emitted when an electron falls from a hydrogen atom's third- to second-lowest energy level.
Why is the wavelength slightly shorter in air?
Index of refraction,n, >1 (vacuum = 1), light slows down by c/n, photon energy is constant therefore frequency remains unchanged, so wavelength is shorter.

Ok, so that just means the wavelength/frequency is changed by air after it has started out at the fundamental 656.46 nm. Makes sense.

by **alter-ego** » Tue Aug 01, 2023 8:56 pm

johnnydeep wrote: ↑Tue Aug 01, 2023 3:24 pm The light of hydrogen link says:

H-alpha (Hα) is a deep-red visible spectral line of the hydrogen atom with a wavelength of 656.28 nm in air and 656.46 nm in vacuum. It is the first spectral line in the Balmer series and is emitted when an electron falls from a hydrogen atom's third- to second-lowest energy level.
Why is the wavelength slightly shorter in air?

Index of refraction,n, >1 (vacuum = 1), light slows down by c/n, photon energy is constant therefore frequency remains unchanged, so wavelength is shorter.

by **johnnydeep** » Tue Aug 01, 2023 3:24 pm

The light of hydrogen link says:

H-alpha (Hα) is a deep-red visible spectral line of the hydrogen atom with a wavelength of 656.28 nm in air and 656.46 nm in vacuum. It is the first spectral line in the Balmer series and is emitted when an electron falls from a hydrogen atom's third- to second-lowest energy level.

Why is the wavelength slightly shorter in air?

by **johnnydeep** » Tue Aug 01, 2023 3:20 pm

De58te wrote: ↑Tue Aug 01, 2023 11:11 am So if the monster was pictured in the light of hydrogen almost 2 weeks ago, would we still see the light of hydrogen today? Or has in the meantime the light switched to something else, say, the light of nitrogen?

Not sure what you're asking. Excited hydrogen is always(?) going to give off H-alpha radiation, and perhaps other radiation energies caused by different electron transitions depending on how excited those electrons were. And if there are any excited nitrogen atoms near the surface of the Sun, we could see them using the appropriate filters. But there's precious little nitrogen, and little else besides hydrogen and helium.

From https://openstax.org/books/astronomy/pa ... of-the-sun :

Code: Select all

Element		% # of Atoms	% Mass
Hydrogen	92.0		73.4
Helium		7.8		25.0
Carbon		0.02		0.20
Nitrogen	0.008		0.09
Oxygen		0.06		0.80
Neon		0.01		0.16
Magnesium	0.003		0.06
Silicon		0.004		0.09
Sulfur		0.002		0.05
Iron		0.003		0.14

by **De58te** » Tue Aug 01, 2023 11:11 am

So if the monster was pictured in the light of hydrogen almost 2 weeks ago, would we still see the light of hydrogen today? Or has in the meantime the light switched to something else, say, the light of nitrogen?

by **APOD Robot** » Tue Aug 01, 2023 4:05 am

Monster Solar Prominence

Explanation: The monsters that live on the Sun are not like us. They are larger than the Earth and made of gas hotter than in any teapot. They have no eyes, but at times, many tentacles. They float. Usually, they slowly change shape and just fade back onto the Sun over about a month. Sometimes, though, they suddenly explode and unleashÂ energetic particles into the Solar System that can attack the Earth.Â Pictured is a huge solar prominence imaged almost two weeks ago in the light of hydrogen. Captured by a small telescope in Gilbert, Arizona, USA, the monster esque plume of gas was held aloft by the ever-present but ever-changing magnetic field near the surface of the Sun. Our active Sun continues to show an unusuallyÂ high numberÂ of prominences, filaments, sunspots, and large active regionsÂ as solar maximum approaches in 2025.

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