<span>In the question,' when you are sitting a few feet from the fire, your skin feels warmed. What form of heat transfer are acting to transfer heat from the fire to your skin, the correct option is A, that is, convection and radiation. Heat transfer is defined as the exchange of thermal energy between physical systems. The rate at which the heat is transfer depends on the temprature of the system and the properties of the intervening medium through which the heat is been transfered. There are three basic modes of heat transfer, these are: conduction, convection and radiation. Conduction is defined as the transfer of heat between two bodies through physical contact. When two bodies which have different temprature come in contact, there will be a transfer of heat energy between them until the two of them have the same temprature. Conduction usually occurs in solids and liquids; it occurs in gases also but it is extremely slow. Convection is the process by which heat is transfer in fluids, that is, liquids and gases. This is how convection operates: when a fluid is heated, it expands and it becomes lighter, this makes it to rise upward and move to the cooler part of the container, as it rises, it will be replaced by the unheated surrounding particles. This cycle continues until heat is evenly distributed all through the fluid. There are two types of convection: natural and forced convection. The heating of the earth surface by the sun ray is an example of natural convection while the air conditioner we use at home operates by mean of forced convection. Both conduction and convection require matter for heat transfer. Radiation is the transfer of heat from one place to another through electromagnetic waves. The hot body transfer heat by emitting electromagnetic waves. The properties of the electromagnetic waves depend on the temperature of the body. The higher the temperature the more intense the rate of emission of radiation. Radiation can occur in all objects and does not require matter for heat transfer. The heat of the sun reaches the earth surface by means of radiation. In the question given, as the air surrounding the fire were heated they rise and were replaced by the unheated air particles. The continuation of this cycle makes the heat energy to be transferred to the objects around. Thus, the heat from the fire was transferred via convection and radiation. </span>
1/ How do scientists determine the elements present in different stars?
KEY WORD for the main idea: Analysis of the starlight.
Answer: Scientists can identify what elements present in different stars by analysing the starlight.
Note: The "starlight" = spectral lines. Astronomers can can identify what kinds of stuff are in stars from the lines they find in star's spectrum. From spectral lines astronomers can determine not only the element, but the temperature and density of that element in the star.
2/ What happens to a star when the fuel for its nuclear fusion reactions is exhausted?
KEY WORD for the main idea: generate light from energy
Answer: A star generates energy through nuclear fusion; this energy is released a light and heat.
Note: The energy source for all stars is nuclear fusion. Stars spend most of their lives repetitively compressing two hydrogen atoms into a single helium atom - plus a lot of energy, which is released as light and heat. A normal star that has exhausted the nuclear fuel in its interior, its outer layers have expanded, and it has become a red giant.
3/ Why aren't stars and galaxies spread out evenly across the universe?
KEY WORD for the main idea: Gravitational attraction
Answer: Stars and Galaxies aren't evenly distributed uniformly throughout the universe, because of the existance of gravity.
Note: Gravity, as one of the fundamental forces of the universe, is the powerful force that glues our universe together. This is the one that causes every object in the universe to attract every other object. One of the most noticeable effects of gravity in the solar system is the orbit of the planets.