APOD: Journey to the Center of the Galaxy (2018 Jul 29)

[APOD Robot]

Journey to the Center of the Galaxy

Explanation: What wonders lie at the center of our Galaxy? In Jules Verne's science fiction classic A Journey to the Center of the Earth, Professor Liedenbrock and his fellow explorers encounter many strange and exciting wonders. Astronomers already know of some of the bizarre objects that exist at our Galactic center, including like vast cosmic dust clouds, bright star clusters, swirling rings of gas, and even a supermassive black hole. Much of the Galactic Center is shielded from our view in visible light by the intervening dust and gas, but it can be explored using other forms of electromagnetic radiation. The featured video is actually a digital zoom into the Milky Way's center which starts by utilizing visible light images from the Digitized Sky Survey. As the movie proceeds, the light shown shifts to dust-penetrating infrared and highlights gas clouds that were recently discovered in 2013 to be falling toward central black hole. In 2018 May, observations of a star passing near the Milky Way's central black hole showed, for the first time, a gravitational redshift of the star's light -- as expected from Einstein's general relativity.

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[zendae1]

That was an amazing ride.
I kept replaying the first 15 seconds or so over and over, and realized that if you focus on the center of the screen, then go right back to the beginning after about 15 seconds and immediately pause it, the Milky Way seems to be receding from you.

[Boomer12k]

That was an amazing ride.
I kept replaying the first 15 seconds or so over and over, and realized that if you focus on the center of the screen, then go right back to the beginning after about 15 seconds and immediately pause it, the Milky Way seems to be receding from you.

This phenomena is called..."Velocitized"...you got used to the motion. When you stopped, it appeared to be reversed. You can see the same effect while driving.

I saw it too with the video...but did not need to rewind to the beginning.
:---[===] *

[star struck]

for those of us non astronomers, how far is the Earth from the center of the galazy?

[heehaw]

for those of us non astronomers, how far is the Earth from the center of the galazy?

26,490 light-years

[rstevenson]

for those of us non astronomers, how far is the Earth from the center of the galazy?

26,490 light-years

Or about 154,275,000 times the minimum length of a circumnavigation of the globe via sailboat (which is, by definition, 40,000 km.) You'd need a very fast sailboat.

Rob

[Sa Ji Tario]

We can say that we will travel for more than 26,000 years at the speed of light, but only for those who see us, to us it will seem instantaneous due to the compression of time

[Chris Peterson]

We can say that we will travel for more than 26,000 years at the speed of light, but only for those who see us, to us it will seem instantaneous due to the compression of time

It's the other way around. Traveling at close to the speed of light, we could make it to the center of the galaxy in a very short time (in our frame). But it could never be less than 26 000 years to those not traveling with us. We might arrive in days, but we'd find that 26 000 years had passed at home.

Edit: Oops... that's what you said. I misunderstood your phrasing.

[Sa Ji Tario]

Friend Chris, The information that light brings to us from the bottom of space does not age and does not change as it travels by logic who travels at the speed of light stays static in "his" time, but for the observer is another thing, therefore, more than 26,000 years will not pass for the traveler (a mi leal saber y entender)

[NateWhilk]

Great video! But I have a question. The time lapse starts in 2004, but in reverse it ends in 2002. Why was the time lapse made that way?

[neufer]

for those of us non astronomers, how far is the Earth from the center of the galazy?

26,490 light-years

Or about 154,275,000 times the minimum length of a circumnavigation of the globe via sailboat (which is, by definition, 40,000 km.) You'd need a very fast sailboat.

More like 6 trillion times. (25,000 miles x 6 trillion = ~25,000 light-years.)

(I think you may have divided by 40,000 TWICE )

[rstevenson]

26,490 light-years

Or about 154,275,000 times the minimum length of a circumnavigation of the globe via sailboat (which is, by definition, 40,000 km.) You'd need a very fast sailboat.

More like 6 trillion times. (25,000 miles x 6 trillion = ~25,000 light-years.)

(I think you may have divided by 40,000 TWICE )

I did it two different ways and got two different answers. I used the one I thought more reasonable. I think the other one was correct. Apparently reasonable doesn't work well with numbers that large, while reason does.

Thanks Art.

Rob

[Sa Ji Tario]

Clipper sailboats were the fastest in history and reached up to 225 km / day, take out the account.

[tomatoherd]

The link "Milky Way's center" to Wiki includes a notation that the diameter of Sgr A* based on angular measurements is around 44 million km, or about Mercury's orbit.
But I always thought black holes were considered geometric points (which of course is impossible, since they contain mass, MUCH mass, even though denser than neutron stars). Was I thinking of the primordial point at time zero of the Big Bang instead? Why did i think that, IOW?

[neufer]

The link "Milky Way's center" to Wiki includes a notation that the diameter of Sgr A* based on angular measurements is around 44 million km, or about Mercury's orbit.

But I always thought black holes were considered geometric points (which of course is impossible, since they contain mass, MUCH mass, even though denser than neutron stars). Was I thinking of the primordial point at time zero of the Big Bang instead? Why did i think that, IOW?

<<The size of a black hole, as determined by the radius of the event horizon, or Schwarzschild radius, is roughly proportional to the mass, M, through

---

where r_s is the Schwarzschild radius and M_Sun is the mass of the Sun. Oppenheimer and his co-authors interpreted the singularity at the boundary of the Schwarzschild radius as indicating that this was the boundary of a bubble in which time stopped. This is a valid point of view for external observers, but not for infalling observers. Because of this property, the collapsed stars were called "frozen stars", because an outside observer would see the surface of the star frozen in time at the instant where its collapse takes it to the Schwarzschild radius.>>

[#Mee Two]

Great video! But I have a question. The time lapse starts in 2004, but in reverse it ends in 2002. Why was the time lapse made that way?

Yeah, and what about data after 2011? I would think 7 more years of observation should show even more interesting stuff, like does the star shift back to blue on the way back around?

[neufer]

The time lapse starts in 2004, but in reverse it ends in 2002. Why was the time lapse made that way?

Yeah, and what about data after 2011? I would think 7 more years of observation should show even more interesting stuff, like does the star shift back to blue on the way back around?

There are actually 3 star shifts to consider:

The 1st order Doppler shift depends linearly upon the radial velocity vis-a-vis Earth : v_r/c


The 2nd order Doppler shift is a purely red shift that maximizes at "periapsis/perimelasma" : v_p²/2c²


The general relativity shift is a purely red shift that maximizes at "periapsis/perimelasma" : ~v_p²/2c²


(Unlike GPS) these two pure red shifts add together at "periapsis/perimelasma": ~v_p²/c² ~ 0.073%

<<Special relativity predicts that the frequency of the atomic clocks moving at GPS orbital speeds will tick more slowly than stationary ground clocks by a factor of:

---

, or result in a delay of about 7 μs/day, where the orbital velocity is v = 4 km/s, and c = the speed of light. The time dilation effect has been measured and verified using the GPS.

The effect of gravitational frequency shift on the GPS due to general relativity is that a clock closer to a massive object will be slower than a clock farther away. Applied to the GPS, the receivers are much closer to Earth than the satellites, causing the GPS clocks to be faster by a factor of 5×10^(−10), or about 45.9 μs/day.>>

[Ann]

Let me repeat Me Two*s question. Does the gravitationally redshifted star become blue(r) again after it has passed the black hole and returned to a part of its orbit that is slightly affected by the black hole's gravitational well?

Ann

[neufer]

Let me repeat Me Two*s question. Does the gravitationally redshifted star become blue(r) again after it has passed the black hole and returned to a part of its orbit that is slightly affected by the black hole's gravitational well?

• Yes(; the gravitational redshift reached a peak but I don't know how much of "the other side of the mountain" was observed ).

[Ann]

Let me repeat Me Two*s question. Does the gravitationally redshifted star become blue(r) again after it has passed the black hole and returned to a part of its orbit that is slightly affected by the black hole's gravitational well?

• Yes(; the gravitational redshift reached a peak but I don't know how much of "the other side of the mountain" was observed ).

Thanks, Art.

Ann

[neufer]

Credit: ESO/MPE/GRAVITY Collaboration

---

Thanks, Art.

Phil Plait has a good article:
http://www.syfy.com/syfywire/a-stars-pl ... etty-smart

Besides the redshift due to the velocity of the S2 as it rounded the black hole, there was also an extra redshift seen due to the black hole’s tremendous gravity equivalent to the redshift produced by an extra 200 km/sec of velocity [= 0.067%]. Credit: ESO/MPE/GRAVITY Collaboration