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Stars

Energy Excess in Nuclei

Nuclear fusion in stars takes elements with a high rest mass per nucleon and transforms them into elements with a low rest mass per nucleon (proton or neutron). In the table given below, the rest mass excess per nucleon is given for elements of different charge and atomic number. The values are given as an energy difference in MeV from the rest mass energy per nucleon of carbon-12.

In this table, the nuclei with charge and atomic numbers that are an integer multiple of helium-4's charge and atomic number are highlighted in red. Of the remaining atoms, those that are intermediate states of the proton-proton process of converting hydrogen into helium are highlighted in green. The remaining atoms that are involved in the CNO cycle for the conversion of hydrogen into helium are highlighted in blue.

This table highlights several points about the elements that impacts nuclear fusion within stars. First, the largest excess is in hydrogen, at 7.3 MeV, and the greatest energy release is in conversion of hydrogen into helium, which releases 6.7 MeV of energy per nucleon. The next point is that elements that are multiples of the helium charge and atomic number are energetically favored over other isotopic states. The final point is that the energy per nucleon in beryllium-8 is slightly less that the energy per nucleon in helium-4, which makes beryllium-8 unstable to decay into two helium atoms. Go to the table.

Rest Mass Excess

A

H

He

Li

Be

B

C

N

O

F

Ne

1

7.289

2

6.567

3

4.983

4.977

4

7.055

0.606

5

6.218

2.291

2.336

6

2.933

2.348

3.063

7

3.719

2.130

2.253

4.000

8

4.000

2.618

0.618

2.865

9

2.773

1.261

1.380

3.221

10

1.261

1.205

1.566

11

1.835

0.789

0.968

12

1.114

0.000

1.447

13

1.274

0.240

0.411

14

0.216

0.205

0.572

15

0.658

0.007

0.191

16

0.355

−0.296

0.682

17

0.463

−0.048

0.115

18

−0.043

0.048

0.296

19

0.175

−0.078

0.092

20

0.190

−0.001

−0.352

21

−0.002

−0.273

22

−0.365

23

−0.224

24

−0.248

A

H

He

Li

Be

B

C

N

O

F

Ne

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