Our Sun —

• The sun is the closest star to our planets. All the other stars are very far away, so they don't seem very bright. (Source1)
• All stars are hot balls of gases, burning up hydrogen gas to produce heat, which is a form of energy. (Source1)
• This is similar to cars using fuels to produce energy for running. To produce heat or do any kind of work, we need energy. (Source1)
• The sun has a surface and an inside central core. Similar to how an avocado has an outer flesh, and a much harder inside.
• When we talk about how hot the sun is, we are actually talking about its surface. When we boil water, we can feel the heat coming from its surface; similarly, on Earth, we can feel the heat coming from the sun's surface.
• The Sun's surface is around 5,500° Celsius/10,000° Fahrenheit. Its core is many times hotter than its surface.

Comparing Sun's temperature with everyday life —

• The temperature of ice is 0° Celsius/32° Fahrenheit.
• The normal temperature of human body is 37° Celsius/98° Fahrenheit.
• The water boils at 100° Celsius/212° Fahrenheit.
• The temperature at which Pizza cooks at pizzeria is 370° — 425° Celsius/700° — 800° Fahrenheit.
• The temperature of a bonfire made during a night out is 1,100° Celsius/2012° Fahrenheit.
• The temperature of a candle flame is 1,400° — 1,650° Celsius/2,552° — 3002° Fahrenheit.
• The temperature of a gas flame is 1,500° Celsius/2732° Fahrenheit.
• So the surface of Sun is roughly 3.5 times hotter than a candle flame and 14 times hotter than a pizza oven. That is why it is impossible for humans to ever get close to surface of the sun.

HOW DO WE KNOW HOW HOT THE SUN IS?

Basics of temperature measurement —

• To measure the hotness or coldness of any object, we need to have a reference. When two students need to compare their heights, they use a large scale.
• In the case of temperature, the scale or reference that we use is called a thermometer.
• Another thing that's important to the measurement of hotness or coldness of any object is a unit. For example, we have meters, kilometers, miles, inches for measuring length; distances, seconds, days, hours for time; and kilograms, tonnes, ounces, for weight.
• For temperature, we use Celsius, Fahrenheit, Kelvin but the widely accepted unit for a temperature is Celsius.
• An important point to remember is that we can measure something in many different units. For example, we can call the time it takes someone to complete a homework as either 30 minutes or half an hour.
• To measure the temperature of any object, we use a thermometer and attach it to the object. The thermometer measures the heat coming from objects and gives a reading.

The temperature of Sun —

• The most basic idea in the science of heat and temperature is that the color of light coming from a source is due to its temperature. 
• For example, the yellow light is less hot compared to blue light. That is why a hot stove which gives a blue flame boils water faster compared to a yellow candle flame of same size.
• Scientists have derived formulas to calculate how much temperatures change with color and they use the formula to measure the temperature of luminous objects (light emitting objects) directly without any thermometer.
• But there is another problem with the sun. The light coming from sun is not actual light. It is a mixture of many lights of different colors.
• Just like when mixing yellow and blue crayons gives green and mixing of red and yellow crayons gives an orange color; similarly, there are so many different colors in sunlight, that they all mix up to become white.
• To measure the temperature using the emitted light, we need to know the exact color. That is why scientists use an instrument called spectrograph. (Source3)
• It is a tool that separates all the colors of sunlight into its base colors and shows them in the form of a band of light. The same thing is also done by raindrops which form rainbows, but scientists can't use rainbows.
• So using the spectrograph, scientists are able to tell the various colors of light coming from the sun and the temperature associated with them.
• Later, they find the average of these different temperatures, which is a simple way of saying "finding the temperature of the middle most color."
• Scientist also use the color of light to find the kind of elements inside the object that emitted the light.
• For the sun, scientists have used this method to find what is inside the sun and what is its internal temperature which depends on many other interesting formulas and theories.