How do we know that Sun is a star?

Hi,
today I will shortly explain how is it that we know that Sun is a star and why it is not so trivial.


When you look up on the clear sky in night you see dots and they are said to be basically the same thing as our close Sun. They definitely shine, but not so much, they are not very warm, light up in different time than Sun, for some reason twinkle and are way smaller, even with binoculars they are still dots.

Hell yeah, they are quite small dots! Even if you take our best telescopes you will always have them so tiny! They are “point source of light”, which means that they are simply so small that from our view and practical purposes they are dimensionless.

They are huge though, most a bit smaller then Sun but still big, but so far away that they seem only as points. If even now we can not see the star’s surface as anything else than a dot, how do we know that it is the same thing as Sun?


The thought of Sun being star was there for some time. I found that already guys in ancient Greece thought that, the same idea came to the famous row of astronomers like Copernicus, Galileo, Kepler, Newton and others but they still did not have an access knowledge that would prove it.

In 1838 Friedrich Bessel measured the distance to star for a first time without considering anything about it (I guess he might have used stellar paralax but I am not sure). He found that the distance is huge, as we know today and soon we calculated that these dots are actually about as bright as Sun, also with astronomical spectroscopy scientists found out that what is happening there is also happening here. To make it clear, we do not have a “close up” image of any star, nor do we have image of any exoplanet.. for now that is simply impossible, still we can admire the cool Jupiter that Juno is taking pics of! (it is literally porn for eyes ūüėČ )

Dragallur

Traveling faster than sound: shockwaves

Hi,
today I want to shortly explain phenomena called “shockwave”.


You may have heard this word already used in the context of supersonic traveling. That is exactly it. Shockwave is the event, whether it is visible or not, that comes when you reach and/or cross the local speed of sound.

I say local because speed of sound changes with temperature, air density and humidity but for normal purposes it is roughly 343.2 meters per second.

When you are slower than the speed of sound the waves made by your movement do not

Circles to illustrate shockwave.

ever hit each other (without obstacle). This you can see on the left first picture. As you move through fluid [1] you create those “circles/ripples” around you and they are closer to each other in the direction you travel.

 

When you speed up to the speed of sound you will create this shockwave because suddenly all of those circles are hitting

everything at the same time which means that the hit is pretty hard. What you see usually is something similar to the picture on the right. This is just the condensed water in the squashed air.

I have read that it is quite dangerous to fly exactly at the speed of sound. It is not very efficient at least because the drag increases 2-3 times compared to supersonic speeds.

With sonic speed you can calculate two numbers. The first one is Mach number which is calculated as your speed divided by speed of sound. This means that Mach 1 is exactly the speed of sound. There is also something called the Mach angle which exists only in supersonic speeds. You can see it labeled as theta in the picture above. The smaller the angle is the faster you travel and the equation goes like this:

sin¬†őł = c/v

Shockwave can also be created in space, though here that speed of sound is way higher (9,000 m/s), I already mentioned this in another post.

For more illustration you can check the video below that I made in GeoGebra:

Dragallur

Read more: 1) 2)

[1]Watch out, fluid means both liquid and gas!

Apollo missions: Apollo 10

Hi,
what a surprise! I am here again with Apollo missions this time with fourth manned Apollo-10-LOGO.pngmission.

Apollo 10 was launched in the year 1969 May 18 and 16:49:00 The Apollo 10 Prime Crew - GPN-2000-001163.jpgPM. Members of crew were three of course: from left to right Cernan, Stafford and Young.

Stafford was captain. Apollo 10 crew were unique because it was first Apollo mission where all members were already in space (Apollo 11 was second and last). Both Young and Stafford were flying for the third time while Cernan for second.

There was also back up crew if some problems occurred but Cooper and Eisele (Apollo 7) were not in favor of NASA because of some incidents on their missions so it was not planned to move them on next mission (as was usual) and they were back up crew only because NASA did not have enough experienced astronauts.

Highlight of this mission were prepared so that they had all data they needed for Apollo 11 which would two months later land on Moon. Everything they needed to do was done, the most important thing was to approach Moon as close as possible.
By one historian it is said that NASA had to give them low amount of fuel so they could not land Moon even if they wanted.

This approaching was achieved by departing a little Lunar module (called Snoopy, it is on the picture with moon) leaving Apollo 10 Lunar Module.jpgonly Young on the board of “Mothership” while Stafford and Cernan achieved the distance from surface of 15.6 km.

Apollo 10 orbited Moon thirty times and then after 8 days and 3 minutes and 23 seconds they splashed down in the Pacific ocean (actually mission was planned 1 minute shorter this means that they really messed it up).

One of Snoopy¬īs stage was left there and nobody knows where it landed (crushed) but in the year of 2011 group of amateurs started project to find it out.

Dragallur

I was taking data from these pages:
https://www.nasa.gov/mission_pages/apollo/missions/apollo10.html#.Vi5Wz7erTc

https://en.wikipedia.org/wiki/Apollo_10

Read about Apollo 9!