[katieHaylor (OP)]

Alastair says:

In a recent episode of The Naked Scientistsyour reporter stated that the sun was made up of clouds of super hot gases. If it is only made of gases, it wouldn’t be very heavy. If it’s not very heavy it wouldn’t have a very big gravitational pull. Therefore, if the earth (or any other star or planet) was able to travel close to the sun,  it would melt, but wouldn’t necessarily be sucked into the sun itself, by gravity.
Is this a correct or a flawed proposition?



Can you help?

[evan_au]

If (the Sun) is only made of gases, it wouldn’t be very heavy.

The Sun is very big, about 1 million km across. It has a volume about 1,300,000 times greater than the Earth.
The Sun does not have to be very dense to mass far more than the Earth!

Although the Sun consists of gas, it is compressed by immense gravitational forces, so that its average density is 1.4 times that of water.

The Sun's core is estimated to have a density about 150 times greater than water, and a temperature around 17 million degrees Celsius.

Strictly speaking, most of the Sun is a "plasma", rather than a gas.
- Familiar gases in Earth's atmosphere are transparent electrical insulators which have molecules bouncing around.
- The Sun's plasma is a glowing electrical conductor which has electrons, protons and alpha particles bouncing around.

if the earth (or any other star or planet) was able to travel close to the sun,  it would melt, but wouldn’t necessarily be sucked into the sun itself, by gravity.

The Earth is in a roughly circular orbit around the Sun.
It takes an enormous amount of energy to launch a space probe to the orbit of Mercury, because you have to cancel much of Earth's orbital velocity to make the probe go closer to the Sun. And then you have to spend even more energy to keep it at Mercury's orbit. The Sun doesn't just automatically "suck it in".

The Parker solar probe (planned launch 2018) is intended to approach within 6 million km of the Sun's visible surface, and it takes even more energy to get this close to the Sun.

To fall into the Sun, a space probe (or comet) would need to pass through a part of the Sun's atmosphere which is dense enough to cancel the enormous orbital velocity that you get by falling into the Sun's gravitational well. In this case, it would be like doing a reentry into Earth's atmosphere, but far faster and far hotter.

[jeffreyH]

For an orbiting craft to move between circular orbits with different radii it has to change velocity twice. To move to a more distant orbit it must speed up and then slow down to the correct velocity to maintain the new orbit. Since the force of gravity has decreased in the process. To do the opposite, move to an orbit nearer the central mass, it must first slow down and then speed up to match the velocity required to maintain its new orbit. This is why what Evan said is so important.

[PmbPhy]

Evan is quite right, as usual. God luv ya Evan!

I'd like to add a bit if I may. When people think of gas they might think of the density of the gas as being close to that of air on earth. The density of air here on Earth is about 1.2 kg/m³. The mean density of the Sun is just a tad higher at 1.4 kg/m³. But as Evan said, there's just a huge amount of it making the total mass of the Sun huge. Since the Sun is nearly spherical the gravitational field outside of the Sun's surface acts like it would if all the mass were at a single point. So one can find gravitational acceleration of the Sun, g, at a distance r (which is the strength of the gravitational field) to be

g = GM/r²

So you can see from here that its only the total mass that determines the strength of a spherically symmetric gravitating source that determines the gravitational field.

Assuming you know a bit of algebra to know what that expression means of course.

[jeffreyH]

Is g a vector?

[evan_au]

The mean density of the Sun is just a tad higher (than Earth's atmosphere) at 1.4 kg/m³.

The average density of the Sun is a lot higher than air, at 1410 kg/m³.
It's even higher than water (1000 kg/m³).
See: http://hyperphysics.phy-astr.gsu.edu/hbase/Solar/sun.html