You've probably heard of a "light-year": that's the distance travelled by a beam of light in 1 year.
Given that light travels at 186,282 miles per second then in a year a beam of light travels
186,282 miles x 60 seconds x 60 minutes x 24 hours x 365.25 days = 5,878,612,843,200 miles
- in words that means a light-year is 5 trillion 878 billion 612 million 843 thousand and two hundred miles.
If we know how far away something is and how fast we can travel it's easy to work out how long it would take to get there: for example a car travelling at 30 mph will take 2 hours to get to a town 60 miles away.
Astronomers can work out roughly how far away stars (and galaxies) are and Engineers can work out how fast their spacecraft can travel, so we can just as easily work out how long it would (theoretically) take to get to places in space.
Clearly the distance to a star in miles = the speed of light in miles per second multiplied by the number of seconds in a year multiplied by the distance to the star in light-years.
We need to write that down as a bit of Maths now and it might look complicated at first glance but it really isn't.
'D' = Distance to a star in miles
'L' = Speed of light in miles per second
'Ly' = Distance to the star in light-years
NOTE: D = L x 3600 x 24 x 365.25 x Ly
Now if 'Ss' = Spaceship speed in mph (I'm using mph because most of us are more familiar with miles per hour than we are with miles per second) then the time to reach that star in years would be the distance to the star in miles divided by the spaceship speed in miles per hour divided by the number of hours in a year, so if we say that 'T' = Time to reach the star in years, then:
T = D / ( Ss x 24 x 365.25)
Now if you glance up to "NOTE" above, we've already worked out that D = L x 3600 x 24 x 365.25 x Ly so we can temporarily replace the value of
'D' with 'L x 3600 x 24 x 365.25 x Ly' making the whole thing look much more complicated (but just for a few seconds) like this:
T = (L x 3600 x 24 x 365.25 x Ly) / (Ss x 24 x 365.25)
Don't panic though because look: '24 x 365.25' appears at the top and the bottom of that awkward-looking equation, so we can remove both figures from the top and bottom and chuck the brackets away at the same time, which leaves us with:
T = L x 3600 x Ly / Ss
Now to make it even more simple because we know that the value of 'L' (the speed of light in miles per second) is 186,282 we can write it down as
T = 186,282 x 3600 x Ly / Ss or even better T = 670,615,200 Ly / Ss
So this is what we've been working towards: the time 'T' in years to travel to a star 'Ly' light-years away at a speed of 'Ss' miles per hour is given by
Now let's stick some actual figures in there to see what this means...
The fastest we've ever managed to propel a spacecraft (The Parker Solar Probe) is approximately 450,000 mph (which is pretty quick you'd think) and the closest star to us is Proxima Centauri, a mere 4.24 light-years away
So there are some practical problems: humans don't last that long and we don't have the capability of building spacecraft with propulsion systems that can travel so far as they'd run out of fuel.
Just for fun, here's another example: the closest spiral galaxy to the Milky Way is 'The Andromeda Galaxy' approximately 2,500,000 light-years away...
- that's over 80% as long as the Earth has existed (and don't forget, the Andromeda Galaxy is in cosmic terms right on our doorstep!)
Play with the equation: find the distance to a star or a galaxy from the internet, decide how fast your spacecraft is going to travel and see how long it would take you to get there.